Modified carbon products, their use in proton exchange membranes and similar devices and methods relating to the same

ABSTRACT

Proton exchange membranes incorporating modified carbon products. The modified carbon products advantageously enhance the properties of proton exchange membranes, leading to more efficiency within a fuel cell or similar device.

CLAIM OF PRIORITY BENEFIT

Pursuant to 35 U.S.C. § 119(e), this patent application claims apriority benefit to: (a) U.S. Provisional Patent Application No.60/553,612 entitled “MODIFIED CARBON PRODUCTS AND THEIR USE IN GASDIFFUSION LAYERS” filed Mar. 15, 2004; (b) U.S. Provisional PatentApplication No. 60/553,413 entitled “MODIFIED CARBON PRODUCTS AND THEIRUSE IN ELECTROCATALYSTS AND ELECTRODE LAYERS” filed Mar. 15, 2004; (c)U.S. Provisional Patent Application No. 60/553,672 entitled “MODIFIEDCARBON PRODUCTS AND THEIR USE IN PROTON EXCHANGE MEMBRANES” filed Mar.15, 2004; and (d) U.S. Provisional Patent Application No. 60/553,611entitled “MODIFIED CARBON PRODUCTS AND THEIR USE IN BIPOLAR PLATES”filed Mar. 15, 2004. This application is also related to U.S. patentapplication Ser. No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIRUSE IN ELECTROCATALYSTS AND ELECTRODE LAYERS AND SIMILAR DEVICES ANDMETHODS RELATING TO THE SAME”, filed on Mar. 15, 2005, and furtheridentified by Attorney File No. 41890-01745, and U.S. patent applicationSer. No. ______, entitled “MODIFIED CARBON PRODUCTS, THEIR USE INFLUID/GAS DIFFUSION LAYERS AND SIMILAR DEVICES AND METHODS RELATING TOTHE SAME”, filed on Mar. 15, 2005, and further identified by AttorneyFile No. 41890-01744, and U.S. patent application Ser. No. ______,entitled “MODIFIED CARBON PRODUCTS, THEIR USE IN BIPOLAR PLATES ANDSIMILAR DEVICES AND METHODS RELATING TO THE SAME”, filed on Mar. 15,2005, and further identified by Attorney File No. 41890-01747. Each ofthe above referenced patent applications is hereby incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production and use of modifiedcarbon products in fuel cell components and similar devices.Specifically, the present invention relates to proton exchange membranesincorporating modified carbon products and methods for making protonexchange membranes including modified carbon products. The modifiedcarbon products can be used to enhance and tailor the properties of theproton exchange membrane.

2. Description of Related Art

Fuel cells are electrochemical devices that are capable of convertingthe energy of a chemical reaction into electrical energy withoutcombustion and with virtually no pollution. Fuel cells are unlikebatteries in that fuel cells convert chemical energy to electricalenergy as the chemical reactants are continuously delivered to the fuelcell. As a result, fuel cells are used to produce a continuous source ofelectrical energy, and compete with other forms of continuous energyproduction such as the combustion engine, nuclear power and coal-firedpower stations. Different types of fuel cells are categorized by theelectrolyte used in the fuel cell. The five main types of fuel cells arealkaline, molten carbonate, phosphoric acid, solid oxide and protonexchange membrane (PEM), also known as polymer electrolyte fuel cells(PEFCs). One particularly useful fuel cell is the proton exchangemembrane fuel cell (PEMFC).

A PEMFC typically includes tens to hundreds of MEAs each of whichincludes a cathode layer and an anode layer. One embodiment of a MEA isillustrated in FIGS. 1(a) and 1(b). One embodiment of a cathode side ofan MEA is also depicted in FIG. 2. With references to FIGS. 1(a), 1(b)and 2, the anode electrocatalyst layer 104 and cathode electrocatalystlayer 106 sandwich a proton exchange membrane 102. In some instances,the combined membrane and electrode layer is referred to as a catalystcoated membrane 103. Power is generated when a fuel (e.g., hydrogen gas)is fed into the anode 104 and oxygen (air) 106 is fed into the cathode.In a reaction typically catalyzed by a platinum-based catalyst in thecatalyst layer of the anode 104, the hydrogen ionizes to form protonsand electrons. The protons are transported through the proton exchangemembrane 102 to a catalyst layer on the opposite side of the membrane(the cathode), where another catalyst, typically platinum or a platinumalloy, catalyzes an oxygen-reduction reaction to form water. Thereactions can be written as follows:Anode: 2H₂→4H⁺+4e ⁻  (1)Cathode: 4H⁺+4e ⁻+O₂→2H₂O  (2)Overall: 2H₂+O₂→2H₂O  (3)

Electrons formed at the anode and cathode are routed through bipolarplates 114 connected to an electrical circuit. On either side of theanode 104 and cathode 106 are porous gas diffusion layers 108, whichgenerally comprise a carbon support layer 107 and a microporous layer109, that help enable the transport of reactants (H₂ and O₂ whenhydrogen gas is the fuel) to the anode and the cathode. On the anodeside, fuel flow channels 110 may be provided for the transport of fuel,while on the cathode side, oxidizer flow channels 112 may be providedfor the transport of an oxidant. These channels may be located in thebipolar plates 114. Finally, cooling water passages 116 can be providedadjacent to or integral with the bipolar plates for cooling the MEA/fuelcell.

A particularly preferred fuel cell for portable applications, due to itscompact construction, power density, efficiency and operatingtemperature, is a PEMFC that can utilize methanol (CH₃OH) directlywithout the use of a fuel reformer to convert the methanol to H₂. Thistype of fuel cell is typically referred to as a direct methanol fuelcell (DMFC). DMFCs are attractive for applications that requirerelatively low power, because the anode reforms the methanol directlyinto hydrogen ions that can be delivered to the cathode through the PEM.Other liquid fuels that may also be used in a fuel cell include formicacid, formaldehyde, ethanol and ethylene glycol.

Like a PEMFC, a DMFC also is made of a plurality of membrane electrodeassemblies (MEAs). A cross-sectional view of a typical MEA isillustrated in FIG. 3 (not to scale). The MEA 300 comprises a PEM 302,an anode electrocatalyst layer 304, cathode electrocatalyst layer 306,fluid distribution layers 308, and bipolar plates 314. Theelectrocatalyst layers 304, 306 sandwich the PEM 302 and catalyze thereactions that generate the protons and electrons to power the fuelcell, as shown below. The fluid diffusion layer 308 distributes thereactants and products to and from the electrocatalyst layers 304, 306.The bipolar plates 314 are disposed between the anode and cathode ofsequential MEA stacks, and comprise current collectors 317 and fuel andoxidizer flow channels, 310, 312, respectively, for directing the flowof incoming reactant fluid to the appropriate electrode. Two end plates(not shown), similar to the bipolar plates, are used to complete thefuel cell stack.

Operation of the DMFC is similar to a hydrogen-gas based PEMFC, exceptthat methanol is supplied to the anode instead of hydrogen gas. Methanolflows through the fuel flow channels 310 of bipolar plate 314, throughthe fluid distribution layer 308 and to the anode electrocatalyst layer304, where it decomposes into carbon dioxide gas, protons and electrons.Oxygen flows through the oxidizer flow channels 312 of the bipolar plate314, through the fluid distribution layer 308, and to the cathodeelectrocatalyst layer, where ionized oxygen is produced. Protons fromthe anode pass through the PEM 302, and recombine with the electrons andionized oxygen to form water. Carbon dioxide is produced at the anode304 and is removed through the exhaust of the cell. The foregoingreactions can be written as follows:Anode: CH₃OH+H₂O→CO₂+6H⁺+6e ⁻  (4)Cathode: 6H⁺+6e ⁻+ 3/2O₂→3H₂O  (5)Overall: 2CH₃OH+3O₂→2CO₂+6H₂O+energy  (6)

There are a number of properties that are required for efficient fuelcell operation. For example, the PEM should have a high protonconductivity to enable efficient transport of protons from the anode tothe cathode, be electrically insulative to prevent transport ofelectrons to the cathode, and act as a robust physical separator. Themembrane should also be impermeable to gases. A common proton exchangematerial currently used is NAFION, a perfluorosulfonic acid (PFSA)polymer available from E.I. duPont deNemours, Wilmington, Del., fromwhich different membrane thicknesses can be formed.

One drawback to current PEMs is that they require a humid fuel tooperate efficiently due to the intermittent spacing of the protonexchange materials within the membrane. Thus, water must be present toact as a bridge between such sites for proton transport. Waterintroduced into and/or produced at the electrodes must be removed fromthe fuel cell to prevent clogging of the electrode pores, a phenomenonknown as “flooding”.

Carbon is a material that has previously been used for some componentsof the fuel cell structure. For example, U.S. Pat. No. 6,280,871 byTosco et al. discloses gas diffusion electrodes containing carbonproducts. The carbon product can be used for at least one component ofthe electrodes, such as the active layer and/or the blocking layer.Methods to extend the service life of the electrodes, as well as methodsto reduce the amount of fluorine-containing compounds are alsodisclosed. Similar products and methods are described in U.S. Pat. No.6,399,202 by Yu et al. Each of the foregoing patents is incorporatedherein by reference in its entirety.

U.S. Patent Application Publication No. 2003/0017379 by Menashi, whichis incorporated herein by reference in its entirety, discloses fuelcells including a gas diffusion electrode, gas diffusioncounter-electrode, and an electrolyte membrane located between theelectrode and counter-electrode. The electrode, counter-electrode, orboth, contain at least one carbon product. The electrolyte membranes canalso contain carbon products. Similar products and methods are describedin U.S. Patent Application Publication No. 2003/0022055 by Menashi,which is also incorporated herein by reference in its entirety.

U.S. Patent Application Publication No. 2003/0124414 by Hertel et al.,which is incorporated herein by reference in its entirety, discloses aporous carbon body for a fuel cell having an electronically conductivehydrophilic agent and discloses a method for the manufacture of thecarbon body. The porous carbon body comprises an electronicallyconductive graphite powder in an amount of between 60 and 80 weightpercent of the body, carbon fiber in an amount of between 5 and 15weight percent of the body, a thermoset binder in an amount between 6and 18 weight percent of the body and an electronically created modifiedcarbon black. Hertel et al. disclose that the carbon body providesincreased wettability without any decrease in electrical conductivity,and can be manufactured without high temperature steps to add graphiteto the body or to incorporate post molding hydrophilic agents into poresof the body.

Composite membranes with increased strength have been studied for use inindustrial chloro-alkali electrolysis processes. Similar fibrilreinforced membranes have been produced for use in PEMFCs. Thesemembranes target the following properties: thin, flat, good mechanicalstrength, chemical robustness and high water permeability and protonconductivity. Such membranes were developed by companies such as W.L.Gore and Associates and Asahi Glass Co., Ltd., all utilizingpolytetrafluoroethylene (PTFE) as the reinforcement agent, and an ionexchange material, such as a sulfonated polymer, to act as the protonconduction sites. Others have utilized inorganic fillers, such assilica, titania or tungstosilicic acid, with solids loadings that rangefrom 5 to 70 weight percent. Fillers such as these have beenincorporated into proton conducting and non-proton conducting polymers.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a proton conductingmembrane including a modified carbon product and a polymer is provided.In one embodiment of the present aspect, the modified carbon product isa modified carbon black. In another embodiment of the present aspect,the modified carbon product is a modified carbon fiber. In yet anotherembodiment, the modified carbon product includes a proton conductingfunctional group. In one embodiment, the proton conducting functionalgroup is selected from the group of SO₃H, CO₂H, PO₃H₂ and PO₃ MH, whereM is a monovalent cation. In another embodiment, the proton conductingfunctional group is selected from the group of carboxylic acids,sulfonic acids, phosphonic acids and phosphonic acid salts. In oneembodiment, the polymer is selected from the group of a sulfonated PTFEand a perfluorosulfonated PTFE. In another embodiment, the polymer isselected from the group of polyvinylidene fluoride (PVDF), acid-doped orderivatized hydrocarbon polymers, such as polybenzimidizole (PBI),polyarylenes, polyetherketones, polysulfones, phosphazenes andpolyimides. In one embodiment, the modified carbon product is coated ona surface of the polymer. In another embodiment, the modified carbonproduct is dispersed within the polymer. In one embodiment, the modifiedcarbon product is adapted to conduct protons in the absence of water. Inanother embodiment, the modified carbon product is adapted toselectively conduct protons in the presence of other hydrogen-comprisingliquid fuels. In yet another embodiment, the modified carbon product isadapted to selectively conduct protons in the presence of methanol orethanol. In one embodiment, the modified carbon product is adapted toyield an increased mechanical strength without a substantial decrease inproton conductivity. In another embodiment, the proton conductingmembrane has a proton conducting group concentration of at least about5.0 millimoles per milliliter, and, in some instances, a protonconducting group concentration of at least about 5.4 millimoles permilliliter. In yet another embodiment, the proton conducting membraneconsists essentially of a modified carbon product.

According to another aspect of the present invention, a method for thefabrication of a proton conducting membrane is provided, the methodincluding the steps of mixing a polymer with a modified carbon productto form a composite mixture and forming the composite mixture into aproton conducting membrane. In one embodiment of the present aspect, theforming step includes extruding the composite mixture. In anotherembodiment, the forming step comprises casting the composite mixture. Inyet another embodiment, the proton conducting membrane has a volumedensity of proton conducting groups of at least about 5.0 millimoles permilliliter. In one embodiment, the composite mixture includes at leastabout 20 weight percent modified carbon product, and, in some instances,includes at least about 40 weight percent modified carbon product. Inone embodiment, the modified carbon product includes carbon black. Inanother embodiment, the modified carbon product includes carbon fibers.

In yet another aspect of the present invention, a method for thefabrication of a proton conducting membrane is provided, the methodincluding the steps of providing a modified carbon black product andforming the modified carbon black product into a thin membrane. In oneembodiment of the present aspect, the forming step includes analogdeposition. In another embodiment of the present aspect, the formingstep includes digital deposition. In yet another embodiment, the formingstep includes dispersing the modified carbon product in a liquid vehicleand ink-jet printing the modified carbon product. In one embodiment, themodified carbon product includes hydrophilic functional groups.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(a) and 1(b) illustrate a schematic cross-section of a PEMFC MEAand bipolar plate assembly according to the prior art.

FIG. 2 illustrates a cross-section of the cathode side of an MEA showingthe membrane and bipolar plate and O₂, H+ and H₂O transport according tothe prior art.

FIG. 3 illustrates a schematic cross-section of a direct methanol fuelcell (DMFC) according to the prior art.

FIG. 4 illustrates a method for modifying a carbon product to formmodified carbon according to U.S. Pat. No. 5,900,029 by Belmont et al.

FIGS. 5(a) and 5(b) illustrate functional groups attached to a carbonsurface according to one via a diazonium salt in accordance with thepresent invention.

FIG. 6 illustrates the increase in active species phase size asprocessing temperature increases.

FIG. 7 illustrates a method for formation of platinum/metal oxide activesites using a modified carbon product according to the presentinvention.

FIG. 8 illustrates the proton conduction mechanism in a PEM according tothe prior art.

FIG. 9 illustrates the proton conduction mechanism in a PEM according anembodiment of the present invention.

FIG. 10 illustrates phosphonic groups incorporated into a PBI membraneaccording to an embodiment of the present invention.

FIG. 11 illustrates the use of sulphonic acid as a proton conductingfunctional group according to an embodiment of the present invention.

FIG. 12 illustrates the use of modified carbon products to decreasecracking during drying as compared to the prior art and according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to fuel cell components that incorporatemodified carbon products. Specifically, the present invention relates toproton exchange membranes incorporating modified carbon products. Theuse of such modified carbon products enables the production of protonexchange membrane (PEMs) having enhanced properties. For example,modified carbon products can be utilized to enhance proton conductivityand electrical insulation properties, as well as physical robustness.

As used herein, a modified carbon product refers to a carbon-containingmaterial having an organic group attached to the carbon surface. In apreferred embodiment, the modified carbon product is a carbon particlehaving an organic group covalently attached to the carbon surface.

A native (unmodified) carbon surface is relatively inert to most organicreactions, and the attachment of specific organic groups at highcoverage levels has been difficult. However, U.S. Pat. No. 5,900,029 byBelmont et al., which is incorporated herein by reference in itsentirety, discloses a process (referred to herein as the Belmontprocess) that significantly improves the ability to modify carbonsurfaces with organic groups. Utilizing the Belmont process, organicgroups can be covalently bonded to the carbon surface such that thegroups are highly stable and do not readily desorb from the carbonsurface.

Generally, the Belmont process includes reacting at least one diazoniumsalt with a carbon material to reduce the diazonium salt, such as byreacting at least one diazonium salt with a carbon black in a proticreaction medium . . . The diazonium salt can include the organic groupto be attached to the carbon. The organic group can be selected from analiphatic group, a cyclic organic group or an organic compound having analiphatic portion and a cyclic portion. The organic group can besubstituted or unsubstituted and can be branched or unbranched.Accordingly, carbon can be modified to alter its properties such as itssurface energy, dispersability in a medium, aggregate size and sizedistribution, dispersion viscosity and/or chemical reactivity.

The modified carbon product can be formed using an electricallyconductive crystalline form of carbon, such as graphite, or can be anamorphous carbon. The carbon, whether crystalline or amorphous, can bein the form of any solid carbon, including carbon black, activatedcarbon, carbon fiber, bulk carbon, carbon cloth, carbon nanotubes,carbon paper, carbon flakes and the like.

It will be appreciated that the carbon material utilized to form themodified carbon product can be selected to suit the specific applicationof the modified carbon product in which the carbon material will beutilized. For example, graphite has an anisotropic plate-like structureand a well-defined crystal structure, resulting in a high electricalconductivity. In one embodiment, a modified carbon product includinggraphite is utilized in a fuel cell component to increase or enhance itselectrical conductivity.

Carbon fibers are long, thin, rod-shaped structures which areadvantageous for physically reinforcing membranes and increasingin-plane electrical conductivity. In one embodiment, modified carbonfibers are utilized in a fuel cell component to increase or maintain itsstructural integrity.

Carbon blacks are homologous to graphite, but typically have arelatively low conductivity and form soft, loose agglomerates ofprimarily nano-sized particles that are isotropic in shape. Carbon blackparticles generally have an average size in the range of 9 to 150nanometers and a surface area of from about 20 to 1500 m²/g. In oneembodiment, a modified carbon product including carbon black is utilizedin the fuel cell component to decrease its electrical conductivity. Inanother embodiment, modified carbon product including carbon black isdispersed in a liquid to form a modified carbon ink that can be utilizedin the production of a fuel cell component due to its shape and smallparticle size.

Generally, a carbon material is modified utilizing the Belmont processvia a functionalizing agent of the form: X—R—Y

-   -   where:        -   X reacts with the carbon surface;        -   R is a linking group; and        -   Y is a functional group.

The functional group (Y) can vary widely, as can the linking group (R),by selection of the appropriate diazonium salt precursor. The diazoniumprecursor has the formula:XN≡NRY

-   -   where:        -   N is nitrogen;        -   X is an anion such as Cl⁻, Br⁻ or F⁻; R is the linking            group; and        -   Y is the functional group.

FIG. 4 schematically illustrates one method of surface modifying acarbon material according to the Belmont process. The carbon material420 is contacted with a diazonium salt 422 to produce a modified carbonproduct 424. The resulting modified carbon product 424 includes surfacegroups that include the linking group (R) and the functional group (Y),as discussed below in relation to FIGS. 5(a) and 5(b).

FIGS. 5(a) and 5(b) illustrate different embodiments of a modifiedcarbon product 524 a, 524 b having a surface group, including afunctional group (Y) and linking group (R) attached to the carbonmaterial. In FIG. 5(a), sulfonic acid is attached to the carbon material520 to produce a modified carbon product 524 a. In FIG. 5(b) polyaminesare attached to the carbon material 520 to produce a modified carbonproduct 524 b.

Examples of functional groups (Y) that can be used to modify the carbonmaterial according to the present invention include those that arecharged (electrostatic), such as sulfonate, carboxylate and tertiaryamine salts. Preferred functional groups for fuel cell componentsaccording to one aspect of the present invention include those thatalter the hydrophobic/hydrophilic nature of the carbon material, such aspolar organic groups and groups containing salts, such as tertiary aminesalts. Particularly preferred hydrophilic functional groups are listedin Table 1, and particularly preferred hydrophobic functional groups arelisted in Table II. TABLE I Hydrophilic Functional Groups (Y) ExamplesCarboxylic acids and salts (C₆H₄)CO₂ ⁻K⁺, (C₆H₄)CO₂H Sulfonic acids andsalts (C₆H₄)CH₂SO₃H Phosphonic acids and salts (C₁₀H₆)PO₃H₂ Amines andamine salts (C₆H₄)NH₃ ⁺Cl⁻ Alcohols (C₆H₄)OH

TABLE II Hydrophobic Functional Groups (Y) Examples Saturated andunsaturated cyclics and (CH₂)₃CH₃, (C₆H₄)CH₃ aliphatics Halogenatedsaturated and unsaturated (C₆H₄)CF₃, (C₆H₄)(CF₂)₇CF₃ cyclics andaliphatics Polymerics Polystyrene [CH₂CH(C₆H₅)]_(n)

According to another aspect, preferred functional groups for fuel cellcomponents are those that increase proton conductivity, such as SO₃H,PO₃H₂ and others known to have good proton conductivity. Particularlypreferred proton conductive functional groups according to the presentinvention are listed in Table III. TABLE III Proton Conducting Groups(Y) Examples Carboxylic acid and salt (C₆H₄)COOH, (C₆H₄)COONa Sulfonicacid and salt (C₆H₄)SO₃H, (C₆H₄)SO₃Na Phosphonic acid and salt(C₆H₄)PO₃H₂, (C₆H₄)PO₃HNa

According to another aspect of the present invention, preferredfunctional groups for fuel cell components include those that increasesteric hindrance and/or physical interaction with other materialsurfaces, such as branched and unbranched polymeric groups. Particularlypreferred polymeric groups according to this aspect are listed in TableIV. TABLE IV Polymeric Groups (Y) Examples Polyacrylate Polymethylmethacrylate (C₆H₄)[CH₂C(CH₃)COOCH₂]_(n) Polystyrene(C₆H₄)[CH(C₆H₅)CH₂]_(n) Polyethylene oxide (PEO)(C₆H₄)[OCH₂CH₂OCH₂CH₂]_(n) Polyethylene glycol (PEG) (C₆H₄)[CH₂CH₂O]nPolypropylene oxide (PPO) (C₆H₄)[OCH(CH₃)CH₂]_(n)

The linking group (R) of the modified carbon product can also vary. Forexample, the linking group can be selected to increase the “reach” ofthe functional group by adding flexibility and degrees of freedom tofurther enhance proton conduction, steric hindrance and/or physicalinteraction with other materials. The linking group can be branched orunbranched. Particularly preferred linking groups according to thepresent invention are listed in Table V. TABLE V Linking Group (R)Examples Alkyls CH₂, C₂H₄ Aryls C₆H₄, C₆H₄CH₂ Cyclics C₆H₁₀, C₅H₄Unsaturated aliphatics CH₂CH═CHCH₂ Halogenated alkyl, aryl, cyclics andC₂F₄, C₆H₄CF₂, C₈F₁₀ unsaturated aliphatics CF₂CH═CHCF₂

Generally, any functional group (Y) can be utilized in conjunction withany linking group (R) to create a modified carbon product for useaccording to the present invention. It will be also appreciated that anyother organic groups listed in U.S. Pat. No. 5,900,029 by Belmont et al.can be utilized in accordance with the present invention.

It will further be appreciated that the modified carbon product caninclude varying amounts of surface groups. The amount of surface groupsin the modified carbon product is generally expressed either on a massbasis (e.g., mmol of surface groups/gram of carbon) or on a surface areabasis (e.g., μmol of surface groups per square meter of carbon materialsurface area). In the latter case, the BET surface area of the carbonsupport material is used to normalize the surface concentration perspecific type of carbon. In one embodiment, the modified carbon producthas a surface group concentration of from about 0.1 μmol/m² to about 6.0μmol/m². In a preferred embodiment, the modified carbon product has asurface group concentration of from about 1.0 μmol/m² to about 4.5μmol/m², and more preferably of from about 1.5 μmol/m² to about 3.0μmol/m².

The modified carbon product can also have more than one functional groupand/or linking group attached to the carbon surface. In one such aspectof the present invention, the modified carbon product includes a secondfunctional group (Y′) attached to the carbon surface. In one embodiment,the second functional group (Y′) is attached to the carbon surface via afirst linking group (R), which also has a first functional group (Y)attached thereto. In another embodiment, the second functional group(Y′) is attached to the carbon surface via a separate second linkinggroup (R′). In this regard, any of the above referenced organic groupscan be attached as the first and/or second organic surface groups, andin any combination.

In one embodiment of the present invention, the modified carbon productsare modified carbon product particles having a well-controlled particlesize. Preferably, the volume average particle size is not greater thanabout 100 μm, more preferably is not greater than about 20 μm and evenmore preferably is not greater than about 10 μm. Further, it ispreferred that the volume average particle size is at least about 0.1μm, more preferably 0.3 μm, even more preferably is at least about 0.5μm and even more preferably is at least about 1 μm. As used herein, theaverage particle size is the median particle size (d₅₀). Powder batcheshaving an average particle size within the preferred parametersdisclosed herein enable the formation of thin layers which areadvantageous for producing energy devices such as fuel cells accordingto the present invention.

In a particular embodiment, the modified carbon product particles have anarrow particle size distribution. For example, it is preferred that atleast about 50 volume percent of the particles have a size of notgreater than about two times the volume average particle size and it ismore preferred that at least about 75 volume percent of the particleshave a size of not greater than about two times the volume averageparticle size. The particle size distribution can be bimodal or trimodalwhich can advantageously provide improved packing density.

In another embodiment, the modified carbon product particles aresubstantially spherical in shape. That is, the particles are preferablynot jagged or irregular in shape. Spherical particles can advantageouslybe deposited using a variety of techniques, including direct writedeposition, and can form layers that are thin and have a high packingdensity, as discussed in further detail below.

Manufacture Of Modified Carbon Products Particles

Modified carbon products useful in accordance with the present inventioncan be manufactured using any known methodology, including, inter alia,the Belmont process, physical adsorption, surface oxidation,sulfonation, grafting, using an alkylating agent in the presence of aFriedel-Crafts type reaction catalyst, mixing benzene and carbon blackwith a Lewis Acid catalyst under anhydrous conditions followed bypolymerization, coupling of a diazotized amine, coupling of onemolecular proportion of a tetrazotized benzidine with two molecularproportions of an arylmethyl pyrazolone in the presence of carbon black,use of an electrochemical reduction of a diazonium salt, and thosedisclosed in and by: Tsubakowa in Polym. Sci., Vol. 17, pp 417470, 1992,U.S. Pat. No. 4,014,844 to Vidal et al., U.S. Pat. No. 3,479,300 toRiven et al., U.S. Pat. No. 3,043,708 to Watson et al., U.S. Pat. No.3,025,259 Watson et al., U.S. Pat. No. 3,335,020 to Borger et al., U.S.Pat. No. 2,502,254 to Glassman, U.S. Pat. No. 2,514,236 to Glassman,U.S. Pat. No. 2,514,236 to Glassman, PCT Patent Application No. WO92/13983 to Centre National De La Recherché Scientifique, and Delmar etal., J. Am. Chem. Soc. 1992, 114, 5883-5884, each of which isincorporated herein by reference in its entirety.

A particularly preferred process for manufacturing modified carbonproduct particles according to the present invention involvesimplementing the Belmont process by spray processing, spray conversionand/or spray pyrolysis, the methods being collectively referred toherein as spray processing. A spray process of this nature is disclosedin commonly-owned U.S. Pat. No. 6,660,680 by Hampden-Smith et al., whichis incorporated herein by reference in its entirety.

Spray processing according to the present invention generally includesthe steps of: providing a liquid precursor suspension, which includes acarbon material and a diazonium salt or a precursor to a diazonium salt;atomizing the precursor to form dispersed liquid precursor droplets; andremoving liquid from the dispersed liquid precursor droplets to form themodified carbon product particles.

Preferably, the spray processing method combines the drying of thediazonium salt and carbon-containing droplets and the conversion of thediazonium precursor salt to a linking group and functional groupcovalently bound to a carbon surface in one step, where both the removalof the solvent and the conversion of the precursor occur essentiallysimultaneously. Combined with a short reaction time, this method enablescontrol over the properties of the linking group and functional groupbound to the carbon surface. In another embodiment, the spray processingmethod achieves the drying of the droplets in a first step, and theconversion of the diazonium salt to a linking group and functional groupin a distinct second step. By varying reaction time, temperature, typeof carbon material and type of precursors, spray processing can producemodified carbon product particles having tailored morphologies andstructures that yield improved performance.

Spray processing advantageously enables the modified carbon productparticles to be formed while the diazonium salt phase is in intimatecontact with the carbon surface, where the diazonium salt is rapidlyreacted on the carbon surface. Preferably, the diazonium salt is exposedto an elevated reaction temperature for not more than about 600 seconds,more preferably not more than about 100 seconds and even more preferablynot more than about 10 seconds.

Spray processing is also capable of forming an aggregate modified carbonproduct particle structure. The aggregate modified carbon productparticles form as a result of the formation and drying of the dropletsduring spray processing, and the properties of the structure areinfluenced by the characteristics of the carbon particles, such as theparticle size, particle size distribution and surface area of the carbonparticles.

Spray processing methods for modified carbon product particlemanufacture according to the present invention can be grouped byreference to several different attributes of the apparatus used to carryout the method. These attributes include: the main gas flow direction(vertical or horizontal); the type of atomizer (submerged ultrasonic,ultrasonic nozzle, two-fluid nozzle, single nozzle pressurized fluid);the type of gas flow (e.g., laminar with no mixing, turbulent with nomixing, co-current of droplets and hot gas, countercurrent of dropletsand gas or mixed flow); the type of heating (e.g., hot wall system, hotgas introduction, combined hot gas and hot wall, plasma or flame); andthe type of collection system (e.g., cyclone, bag house, electrostaticor settling).

For example, modified carbon product particles can be prepared bystarting with a precursor liquid including a protic reaction medium(e.g., an aqueous-based liquid), colloidal carbon and a diazonium salt.The processing temperature of the precursor droplets can be controlledso the diazonium salt reacts, leaving the carbon intact but surfacefunctionalized. The precursor liquid may also or alternatively includean aprotic reaction medium such as acetone, dimethyl formamide, dioxaneand the like.

The atomization technique has a significant influence over thecharacteristics of the modified carbon product particles, such as thespread of the particle size distribution (PSD), as well as theproduction rate of the particles. In extreme cases, some techniquescannot atomize precursor compositions having only moderate carbonparticle loading or high viscosities. Several methods exist for theatomization of precursor compositions containing suspended carbonparticulates. These methods include, but are not limited to: ultrasonictransducers (usually at a frequency of 1-3 MHz); ultrasonic nozzles(usually at a frequency of 10-150 KHz); rotary atomizers; two-fluidnozzles; and pressure atomizers.

Ultrasonic transducers are generally submerged in a liquid, and theultrasonic energy produces atomized droplets on the surface of theliquid. Two basic ultrasonic transducer disc configurations, planar andpoint source, can be used. Deeper fluid levels can be atomized using apoint source configuration since the energy is focused at a point thatis some distance above the surface of the transducer. The scale-up ofsubmerged ultrasonic transducers can be accomplished by placing a largenumber of ultrasonic transducers in an array. Such a system isillustrated in U.S. Pat. No. 6,103,393 by Kodas et al. and U.S. Pat. No.6,338,809 by Hampden-Smith et al., each of which is incorporated hereinby reference in its entirety.

Spray nozzles can also be used, and the scale-up of nozzle systems canbe accomplished by either selecting a nozzle with a larger capacity, orby increasing the number of nozzles used in parallel. Typically, thedroplets produced by nozzles are larger than those produced byultrasonic transducers. Particle size is also dependent on the gas flowrate. For a fixed liquid flow rate, an increased airflow decreases theaverage droplet size and a decreased airflow increases the averagedroplet size. It is difficult to change droplet size without varying theliquid or airflow rates. However, two-fluid nozzles have the ability toprocess larger volumes of liquid per unit time than ultrasonictransducers.

Ultrasonic spray nozzles use high frequency energy to atomize a fluidand have some advantages over single or two-fluid nozzles, such as thelow velocity of the spray leaving the nozzle and lack of associated gasflow. The nozzles are available with various orifice sizes and orificediameters that allow the system to be scaled for the desired productioncapacity. In general, higher frequency nozzles are physically smaller,produce smaller droplets, and have a lower flow capacity than nozzlesthat operate at lower frequencies. A drawback of ultrasonic nozzlesystems is that scaling up the process by increasing the nozzle sizeincreases the average particle size. If a particular modified carbonproduct particle size is required, then the maximum production rate pernozzle is set. If the desired production rate exceeds the maximumproduction rate of the nozzle, additional nozzles or additionalproduction units will be required to achieve the desired productionrate.

The shape of the atomizing surface determines the shape and spread ofthe spray pattern. Conical, microspray and flat atomizing surface shapesare available. The conical atomizing surface provides the greatestatomizing capability and has a large spray envelope. The flat atomizingsurface provides almost as much flow as the conical, but limits theoverall diameter of the spray. The microspray atomizing surface is forvery low flow rates where narrow spray patterns are needed. Thesenozzles are preferred for configurations where minimal gas flow isrequired in association with the droplets.

Particulate suspensions present several problems with respect toatomization. For example, submerged ultrasonic atomizers re-circulatethe suspension through the generation chamber and the suspensionconcentrates over time. Further, some fraction of the liquid atomizeswithout carrying the suspended carbon particulates. When using submergedultrasonic transducers, the transducer discs can become coated with theparticles over time. Further, the generation rate of particulatesuspensions is very low using submerged ultrasonic transducer discs, duein part to energy being absorbed or reflected by the suspendedparticles.

For spray drying, an aerosol can be generated using three basic methods.These methods differ in the type of energy used to break the liquidmasses into small droplets. Rotary atomizers (utilization of centrifugalenergy) make use of spinning liquid droplets off of a rotating wheel ordisc. Rotary atomizers are useful for co-current production of dropletsin the range of 20 to 150 μm in diameter. Pressure nozzles (utilizationof pressure energy) generate droplets by passing a fluid under highpressure through an orifice. These can be used for both co-current andmixed-flow reactor configurations, and typically produce droplets in thesize range of 50 to 300 μm. Multiple fluid nozzles, such as a two fluidnozzle, produce droplets by passing a relatively slow moving fluidthrough an orifice while shearing the fluid stream with a relativelyfast moving gas stream. As with pressure nozzles, multiple fluid nozzlescan be used with both co-current and mixed-flow spray dryerconfigurations. This type of nozzle can typically produce droplets inthe range of 5 to 200 μm.

For example, two-fluid nozzles are used to produce aerosol sprays inmany commercial applications, typically in conjunction with spray dryingprocesses. In a two-fluid nozzle, a low-velocity liquid streamencounters a high-velocity gas stream that generates high shear forcesto accomplish atomization of the liquid. A direct result of thisinteraction is that the droplet size characteristics of the aerosol aredependent on the relative mass flow rates of the liquid precursor andnozzle gas stream. The velocity of the droplets as they leave thegeneration zone can be quite large which may lead to unacceptable lossesdue to impaction. The aerosol also leaves the nozzle in a characteristicpattern, typically a flat fan, and this may require that the dimensionsof the reactor be sufficiently large to prevent unwanted losses on thewalls of the system.

The next step in the process includes the evaporation of the solvent(typically water) as the droplet is heated, resulting in a carbonparticle of dried solids and salts. A number of methods to deliver heatto the particle are possible: horizontal hot-wall tubular reactors,spray drier and vertical tubular reactors can be used, as well asplasma, flame and laser reactors. As the carbon particles experienceeither higher temperature or longer time at a specific temperature, thediazonium salt reacts. Preferably, the temperature and amount of timethat the droplets/particles experience can be controlled, and,therefore, the properties of the linking group and functional groupformed on the carbon surface can also be controlled.

For example, a horizontal, tubular hot-wall reactor can be used to heata gas stream to a desired temperature. Energy is delivered to the systemby maintaining a fixed boundary temperature at the wall of the reactorand the maximum temperature of the gas is the wall temperature. Heattransfer within a hot wall reactor occurs through the bulk of the gasand buoyant forces that occur naturally in horizontal hot wall reactorsaid this transfer. The mixing also helps to improve the radialhomogeneity of the gas stream. Passive or active mixing of the gas canalso increase the heat transfer rate. The maximum temperature and theheating rate can be controlled independent of the inlet stream withsmall changes in residence time. The heating rate of the inlet streamcan also be controlled using a multi-zone furnace.

The use of a horizontal hot-wall reactor according to the presentinvention is preferred to produce modified carbon product particles witha size of not greater than about 5 μm. One disadvantage of such reactorsis the poor ability to atomize carbon particles when using submergedultrasonics for atomization.

Alternatively, a horizontal hot-wall reactor can be used with atwo-fluid nozzle. This method is preferred for precursor feed streamscontaining relatively high levels of carbon. A horizontal hot-wallreactor can also be used with ultrasonic nozzles, which allowsatomization of precursors containing particulate carbons. However, largedroplet size can lead to material loss on reactor walls and othersurfaces, making this an expensive method for production of modifiedcarbon product particles.

While horizontal hot-wall reactors are useful according to the presentinvention, spray processing systems in the configuration of a spraydryer are the generally preferred production method for large quantitiesof modified carbon product particles. Spray drying is a process whereparticles are produced by atomizing a precursor to produce droplets andevaporating the liquid to produce a dry aerosol, where thermaldecomposition of one or more precursors (e.g., a carbon and/or diazoniumsalt) may take place to produce the particle. The residence time in thespray dryer is the average time the process gas spends in the dryingvessel as calculated by the vessel volume divided by the process gasflow using the outlet gas conditions. The peak excursion temperature(i.e., the reaction temperature) in the spray dryer is the maximumtemperature of a particle, averaged throughout its diameter, while theparticle is being processed and/or dried. The droplets are heated bysupplying a pre-heated carrier gas.

Three types of spray dryer systems are useful for spray drying to formmodified carbon product particles according to the present invention. Anopen system is useful for general spray drying to form modified carbonproduct particles using air as an aerosol carrier gas and an aqueousfeed solution as a precursor. A closed system is useful for spray dryingto form modified carbon product particles using an aerosol carrier gasother than air. A closed system is also useful when using a non-aqueousor a semi-non-aqueous solution as a precursor. A semi-closed system,including a self-inertizing system, is useful for spray drying to formmodified carbon product particles that require an inert atmosphereand/or precursors that are potentially flammable.

Two spray dryer designs are particularly useful for the production ofmodified carbon product particles according to the present invention. Aco-current spray dryer is useful for production of modified carbonproduct particles that are sensitive to high temperature excursions(e.g., greater than about 350° C.), or that require a rotary atomizer togenerate the aerosol. Mixed-flow spray dryers are useful for producingmodified carbon product particles that require relatively hightemperature excursions (e.g., greater than about 350° C.), or requireturbulent mixing forces. According one embodiment of the presentinvention, co-current spray-drying is preferred for the manufacture ofmodified carbon product particles, including modified carbon black.

In a co-current spray dryer, the hot gas is introduced at the top of theunit, where the droplets are generated with any of the above-describedatomization techniques. Generally, the maximum temperature that adroplet/particle is exposed to in a co-current spray dryer is thetemperature at the outlet of the dryer. Typically, this outlettemperature is limited to about 200° C., although some designs allow forhigher temperatures. In addition, since the particles experience thelowest temperature in the beginning of the time-temperature curve andthe highest temperature at the end, the possibility of precursor surfacediffusion and agglomeration is high.

A mixed-flow spray dryer introduces the hot gas at the top of the unitwhile precursor droplets are generated near the bottom and directedupwardly. The droplets/particles are forced towards the top of the unit,and then fall and flow back down with the gas, increasing the residencetime in the spray dryer. The temperature experienced by thedroplets/particles is higher compared to a co-current spray dryer.

These conditions are advantageous for the production of modified carbonproduct particles having a wide range of surface group concentrationsincluding surface concentrations up to 6 μmol/m² organic groups oncarbon. For co-current spray dryers the reaction temperatures can behigh enough to enable reaction of the diazonium salt (e.g., between 25°C. and 100° C.). The highest temperature in co-current spray dryers isthe inlet temperature (e.g., 180° C.), and the outlet temperature can beas low as 50° C. Therefore, the carbon particles and surface groupsreach the highest temperature for a relatively short time, whichadvantageously reduces migration or surface diffusion of the surfacegroups. This spike of high temperature can also quickly convert thediazonium salt to the bonded surface group, and is followed by a mildquench since the spray dryer temperature quickly decreases after themaximum temperature is achieved. Thus, the spike-like temperatureprofile can be advantageous for the generation of highly dispersedsurface groups on the surface of the carbon.

The range of useful residence times for producing modified carbonproduct particles depends on the spray dryer design type, atmosphereused, nozzle configuration, feed liquid inlet temperature and theresidual moisture content. In general, residence times for theproduction of modified carbon product particles can range from less than3 seconds up to 5 minutes.

For a co-current spray-drying configuration, the range of useful inlettemperatures for producing modified carbon product particles depends ona number of factors, including solids loading and droplet size,atmosphere used and energy required to perform drying and/or reaction ofthe diazonium salt. Useful inlet temperatures should be sufficientlyhigh to accomplish the drying and/or reaction of the diazonium saltwithout promoting significant surface diffusion of the surface groups.

In general, the outlet temperature of the spray dryer determines theresidual moisture content of the modified carbon product particles. Forexample, a useful outlet temperature for co-current spray dryingaccording to one embodiment of the present invention is from about 50°C. to about 80° C. Useful inlet temperatures according to the presentinvention are from about 130° C. to 180° C. The carbon solids (e.g.,particulate) loading can be up to about 50 wt. %.

Other equipment that is desirable for producing modified carbon productparticles using a spray dryer includes a heater for heating the gas,directly or indirectly, including by thermal, electrical conductive,convective and/or radiant heating. Collection apparatus, such ascyclones, bag/cartridge filters, electrostatic precipitators, and/orvarious wet collection apparatus, may also be utilized to collect themodified carbon product particles.

In one embodiment of the present invention, spray drying is used to formaggregate modified carbon product particles, wherein the aggregatesinclude more than one modified carbon product particle. In this regard,the individual modified carbon product particles can all haveessentially the same surface groups or varying types of modified carbonproduct particles can be utilized to provide an aggregate with a mixtureof surface groups. For example, a first modified carbon product particlewithin the aggregate can have a hydrophilic surface group and a secondmodified carbon product particle can have a hydrophobic surface group.

In one aspect, first modified carbon product particles (e.g., modifiedcarbon black particles having a hydrophilic surface group) and secondmodified carbon product particles (e.g., modified carbon black particleshaving a hydrophobic surface group) are dispersed in a aqueous precursorsolution and spray dried to obtain an aggregate modified carbon productparticle having both hydrophilic and hydrophobic properties. Theaggregate may include various particle sizes, from nano-sized particlesto large, sub-micron size particles.

Moreover, as described below with respect to electrocatalyst materials,the aggregate structure can include smaller primary carbon particles andtwo or more types of primary particles can be mixed. For example, two ormore types of particulate carbon (e.g., amorphous and graphitic carbon)can be combined within the aggregate to tailor the aggregate to thedesired electrical and/or oxidation resistant properties.

In this regard, spray drying techniques can be used simply to form theaggregate modified carbon product particles, or to additionally effect achange in the structure of the individual modified carbon productparticles. For example, spray processing techniques can be conducted athigher temperatures to effect at least a partial decomposition of thepreviously attached surface groups, such as those surface groups thatare utilized to help the spray processing, but are subsequently notdesired in the end-product. The specific temperature for the spraydrying process may be chosen depending on the desired outcome, which isa function of the type and stability of the surface groups, the targetedfinal composition, and the treatment distribution.

Electrocatalyst Materials

Electrocatalysts are used in the fuel cell to facilitate the desiredreactions. Particularly preferred electrocatalyst materials useful inaccordance with the present invention include those having an activespecies phase, such as a metal, dispersed on a support phase, such as acarbon material. Such electrocatalyst materials are described in U.S.Pat. No. 6,660,680 by Hampden-Smith et al. As used herein, the terms“electrocatalyst materials”, “electrocatalyst particles” and/or“electrocatalyst powders” and the like refer to such electrocatalystmaterials in a non-modified native state.

With respect to electrocatalyst materials, the larger structures formedfrom the association of discrete carbon particles supporting thedispersed active species phase are referred to as aggregates oraggregate particles, and typically have a size in the range from 0.3 to100 mm. In addition, the aggregates can further associate into larger“agglomerates”. The aggregate morphology, aggregate size, sizedistribution and surface area of the electrocatalyst powders are allcharacteristics that impact the catalyst performance. The aggregatemorphology, aggregate size and size distribution determines the packingdensity, and the surface area determines the type and number of surfaceadsorption centers where the active species form during synthesis of theelectrocatalyst.

The aggregate structure can include smaller primary carbon particles,constituting the support phase. Two or more types of primary particlescan be mixed to form the support phase. For example, two or more typesof particulate carbon (e.g., amorphous and graphitic carbon) can becombined to form the support phase. The two types of particulate carboncan have different performance characteristics and the combination ofthe two types in the aggregate structure can enhance the performance ofthe catalyst.

The carbon support is a major component of the electrocatalysts. Toachieve adequate dispersion of the active sites, the carbon supportshould have a high surface area, a large accessible porous surface area(pore sizes from about 2 nm to about 50 nm preferred), low levels ofcontaminants that are poisons for either the membrane or the activesites during long term operation of the fuel cell, and good stabilitywith respect to oxidation during the operation of the fuel cell.

Among the forms of carbon available for the support phase, graphiticcarbon is preferred for long-term operational stability of fuel cellsdue to its ability to resist oxidation. Amorphous carbon (e.g., carbonblack) is preferred when a smaller crystallite size is desired for thesupported active species phase. The carbon support particles typicallyhave sizes in the range of from about 10 nanometers to 5 μm, dependingon the nature of the carbon material. However, carbon particulateshaving sizes up to 25 μm can also be used.

The compositions and ratios of the aggregate particle components can beindependently varied, and various combinations of carbons, metals, metalalloys, metal oxides, mixed metal oxides, organometallic compounds andtheir partial pyrolysis products can be used. The electrocatalystparticles can include two or more different materials as the dispersedactive species. As an example, combinations of Ag and MnO_(x) dispersedon carbon can be useful for some electrocatalytic applications. Otherexamples of multiple active species are mixtures of metal porphyrins,partially decomposed metal porphyrins, Co and CoO.

The supported electrocatalyst particles preferably include a carbonsupport phase with at least about 1 weight percent active species phase,more preferably at least about 5 weight percent active species phase andeven more preferably at least about 10 weight percent active speciesphase. In one embodiment, the particles include from about 20 to about80 weight percent of the active species phase dispersed on the supportphase. It has been found that such compositional levels give rise to themost advantageous electrocatalyst properties for many applications.However, the preferred level of the active species supported on thecarbon support will depend upon the total surface area of the carbon,the type of active species phase and the application of theelectrocatalyst. A carbon support having a low surface area will requirea lower percentage of active species on its surface to achieve a similarsurface concentration of the active species compared to a support withhigher surface area and higher active species loading.

Metal-carbon electrocatalyst particles include a catalytically activespecies of at least a first metal phase dispersed on a carbon supportphase. The metal active species phase can include any metal and theparticularly preferred metal will depend upon the application of thepowder. The metal phase can be a metal alloy wherein a first metal isalloyed with one or more alloying elements. As used herein, the termmetal alloy also includes intermetallic compounds between two or moremetals. For example, the term platinum metal phase refers to a platinumalloy or platinum-containing intermetallic compound, as well as pureplatinum metal. The metal-carbon electrocatalyst powders can alsoinclude two or more metals dispersed on the support phase as separateactive species phases.

Preferred metals for the active species include the platinum groupmetals and noble metals, particularly Pt, Ag, Pd, Ru, Os and theiralloys. The metal phase can also include a metal selected from the groupconsisting of Ni, Rh, Ir, Co, Cr, Mo, W, V, Nb, Al, Ta, Ti, Zr, Hf, Zn,Fe, Cu, Ga, In, Si, Ge, Sn, Y, La, lanthanide metals and combinations oralloys of these metals. Preferred metal alloys include alloys of Pt withother metals, such as Ru, Os, Cr, Ni, Mn and Co. Particularly preferredamong these is Pt or PtRu for use in the anode and Pt, PtCrCo or PtNiCofor use in the cathode.

Alternatively, metal oxide-carbon electrocatalyst particles that includea metal oxide active species dispersed on a carbon support phase can beused. The metal oxide can be selected from the oxides of the transitionmetals, preferably those existing in oxides of variable oxidationstates, and most preferably from those having an oxygen deficiency intheir crystalline structure. For example, the metal oxide active speciescan be an oxide of a metal selected from the group consisting of Au, Ag,Pt, Pd, Ni, Co, Rh, Ru, Fe, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti orAl. A particularly preferred metal oxide active species is manganeseoxide (MnO_(x), where x is 1 to 2). The active species can include amixture of different oxides, solid solutions of two or more differentmetal oxides or double oxides. The metal oxides can be stoichiometric ornon-stoichiometric and can be mixtures of oxides of one metal havingdifferent oxidation states. The metal oxides can also be amorphous.

It is preferred that the average size of the active species is such thatthe electrocatalyst particles include small single crystals orcrystallite clusters, collectively referred to herein as clusters, ofthe active species dispersed on the support phase. Preferably, theaverage active species cluster size (diameter) is not greater than about10 nanometers, more preferably is not greater than about 5 nanometersand even more preferably is not greater than about 3 nanometers.Preferably, the average cluster size of the active species is from about0.5 to 5 nanometers. Preferably, at least about 50 percent by number,more preferably at least about 60 percent by number and even morepreferably at least about 70 percent by number of the active speciesphase clusters have a size of not greater than about 3 nanometers.Electrocatalyst powders having a dispersed active species phase withsuch small crystallite clusters advantageously have enhanced catalyticproperties as compared to powders including an active species phasehaving larger clusters.

It should be recognized that the preferred electrocatalyst powders arenot mere physical admixtures of different particles, but are comprisedof support phase particles that include a dispersed phase of an activespecies. Preferably, the composition of the aggregate electrocatalystparticles is homogeneous. That is, the different phases of theelectrocatalyst are well dispersed within a single aggregate particle.It is also possible to intentionally provide compositional gradientswithin the individual electrocatalyst aggregate particles. For example,the concentration of the dispersed active species phase in a compositeparticle can be higher or lower at the surface of the secondary supportphase than near the center and gradients corresponding to compositionalchanges of 10 to 100 weight percent can be obtained. When the aggregateparticles are deposited using a direct-write tool, the aggregateparticles preferably retain their structural morphology and thereforethe functionality of the compositional gradient can be exploited in thedevice.

In addition, the electrocatalyst powders preferably have a surface areaof at least about 25 m²/g, more preferably at least about 90 m²/g andeven more preferably at least about 600 m²/g. Surface area is typicallymeasured using the BET nitrogen adsorption method which is indicative ofthe surface area of the powder, including the surface area of accessiblepores on the surface of the particles.

Moreover, many of the desired attributes of modified carbon products maybe desired attributes of electrocatalyst, and any of the above-describedattributes of the modified carbon products can be acknowledged as beinguseful in the production, use and application of electrocatalystmaterials. For example, particle size, size distribution and sphericalnature can be an important factor when utilizing such electrocatalystmaterials in an electrocatalyst ink, as described in further detailbelow.

Manufacture of Electrocatalyst Materials

Electrocatalyst materials may be produced in a variety of ways includingimpregnation and co-precipitation. One preferred method for preparingparticulate electrocatalyst materials is by spray processing, oneapproach of which is disclosed in U.S. Pat. No. 6,660,680 toHampden-Smith et al.

Production of electrocatalyst material by spray processing generallyinvolves the steps of: providing a precursor composition which includesa support phase or a precursor to the support phase (e.g., acarbon-containing material) and a precursor to the active species;atomizing the precursor to form a suspension of liquid precursordroplets; and removing liquid from liquid precursor droplets to form thepowder. At least one component of the liquid precursor is chemicallyconverted into a desired component of the powder. The drying of theprecursors and the conversion to a catalytically active species can becombined in one step, where both the removal of the solvent and theconversion of a precursor to the active species occur essentiallysimultaneously. Combined with a short reaction time, this enablescontrol over the distribution of the active species on the support, theoxidation state of the active species and the crystallinity of theactive species. By varying reaction time, temperature, type of supportmaterial and type of precursors, electrocatalyst materials havingwell-controlled catalyst morphologies and active species structures canbe produced, which yield improved catalytic performance.

The precursor composition can include low temperature precursors, suchas a molecular metal precursor that has a relatively low decompositiontemperature. As used herein, the term molecular metal precursor refersto a molecular compound that includes a metal atom. Examples includeorganometallics (molecules with carbon-metal bonds), metal organics(molecules containing organic ligands with metal bonds to other types ofelements such as oxygen, nitrogen or sulfur) and inorganic compoundssuch as metal nitrates, metal halides and other metal salts. Themolecular metal precursors can be either soluble or insoluble in theprecursor composition.

In general, molecular metal precursor compounds that eliminate ligandsby a radical mechanism upon conversion to metal are preferred,especially if the species formed are stable radicals, and, therefore,lower the decomposition temperature of that precursor compound.

Furthermore, molecular metal precursors containing ligands thateliminate cleanly upon precursor conversion are preferred because theyare not susceptible to carbon contamination or contamination by anionicspecies such as nitrates. Therefore, preferred precursors for metalsinclude carboxylates, alkoxides or combinations thereof that convert tometals, metal oxides or mixed metal oxides by eliminating smallmolecules such as carboxylic acid anhydrides, ethers or esters.

Particularly preferred molecular metal precursor compounds are metalprecursor compounds containing silver, nickel, platinum, gold,palladium, copper, ruthenium, cobalt and chromium. In one preferredembodiment of the present invention, the molecular metal precursorcompound comprises platinum.

Various molecular metal precursors can be used for platinum metal.Preferred molecular precursors for platinum include nitrates,carboxylates, beta-diketonates, and compounds containing metal-carbonbonds. Divalent platinum (II) complexes are particularly preferred.Preferred molecular precursors also include ammonium salts of platinatessuch as ammonium hexachloro platinate (NH₄)₂PtCl₆, and ammoniumtetrachloro platinate (NH₄)₂PtCl₄; sodium and potassium salts ofhalogeno, pseudohalogeno or nitrito platinates such as potassiumhexachloro platinate K₂PtCl₆, sodium tetrachloro platinate Na₂PtCl₄,potassium hexabromo platinate K₂PtBr₆, potassium tetranitrito platinateK₂Pt(NO₂)₄; dihydrogen salts of hydroxo or halogeno platinates such ashexachloro platinic acid H₂PtCl₆, hexabromo platinic acid H₂PtBr₆,dihydrogen hexahydroxo platinate H₂Pt(OH)₆; diammine and tetraammineplatinum compounds such as diammine platinum chloride Pt(NH₃)₂Cl₂,tetraammine platinum chloride [Pt(NH₃)₄]Cl₂, tetraammine platinumhydroxide [Pt(NH₃)₄](OH)₂, tetraammine platinum nitrite[Pt(NH₃)₄](NO₂)₂, tetrammine platinum nitrate Pt(NH₃)₄(NO₃)₂, tetrammineplatinum bicarbonate [Pt(NH₃)₄](HCO₃)₂, tetraammine platinumtetrachloroplatinate [Pt(NH₃)₄]PtCl₄; platinum diketonates such asplatinum (II) 2,4-pentanedionate Pt(C₅H₇O₂)₂; platinum nitrates such asdihydrogen hexahydroxo platinate H₂Pt(OH)₆ acidified with nitric acid;other platinum salts such as Pt-sulfite and Pt-oxalate; and platinumsalts comprising other N-donor ligands such as [Pt(CN)₆]₄ ⁺.

Modified Electrocatalyst Products

According to one embodiment of the present invention, the modifiedelectrocatalyst products are a subclass of the above-described modifiedcarbon products, and as used herein, modified electrocatalyst productsgenerally refers to an electrocatalyst material having an organic groupattached thereto.

In one embodiment of the present invention, a modified electrocatalystproduct is provided having an active species phase, a carbon supportphase, and an organic surface group covalently bonded to the carbonsupport phase.

In one preferred embodiment, the active species phase includes a firstmetal, such as platinum. The active species phase can also include asecond metal, such as ruthenium, cobalt, chromium or nickel. The firstand second metals can be in metallic, metal oxide or alloy form, asdescribed in further detail below. In yet another embodiment, the activespecies phase includes at least three metals (e.g., Pt, Ni and Co). Theactive species phase may be any of the above-mentioned metals or metaloxides utilized in the above-described electrocatalyst materials.

The carbon support material can be any of the above-described materialsutilized in a modified carbon product or electrocatalyst material. Inone preferred embodiment, the carbon support material is carbon black.

The organic group may include aliphatic groups, cyclic organic groupsand organic compounds having an aliphatic portion and a cyclic portion.The organic group can be substituted or unsubstituted and can bebranched or unbranched. Generally, as described above, the organicgroups include a linking group (R) and a functional group (Y), moregenerally known as surface groups.

Any of the above-described functional groups (Y) utilized to form amodified carbon product can also be used in the production of a modifiedelectrocatalyst product, including those that are charged(electrostatic), such as sulfonate, carboxylate and tertiary aminesalts. Preferred functional groups include those that alter thehydrophobic or hydrophilic nature of the carbon material, such as polarorganic groups and groups containing salts, such as tertiary aminesalts, including those listed in Tables I and II. Another particularlypreferred class of functional groups include those that increase protonconductivity, such as SO₃H, PO₃H₂ and others known to be part of thebackbone of a proton conducting membrane, including those listed inTable III. Yet another particularly preferred class of functional groupsincludes compounds that increase steric hindrance and/or physicalinteraction with other material surfaces, such as such as those listedin Table IV.

Any of the linking groups (R) utilized in the creation of a modifiedcarbon product can also be used in the production of a modifiedelectrocatalyst products, including those that increase the “reach” ofthe functional group by adding flexibility and degrees of freedom tofurther increase, for example, proton conduction, steric hindranceand/or physical and/or interaction with other materials, includingbranched and unbranched materials. Particularly preferred linking groupsare listed in Table V, above.

It will be appreciated that, generally, any functional group (Y) can beutilized in conjunction with any linking group (R) to create a modifiedelectrocatalyst product according to the present invention to producethe desired effect within the fuel cell component. It will be alsoappreciated that any other organic groups disclosed in U.S. Pat. No.5,900,029 by Belmont et al. can be utilized.

As noted above, modified electrocatalyst products are a subclass ofmodified carbon products. Thus, many of the desired attributes ofmodified carbon products are also desired attributes of modifiedelectrocatalyst products, and any of the above-described attributes ofmodified carbon products can be acknowledged as being useful in theproduction, use and application of modified electrocatalyst products.For example, particle size, size distribution and spherical nature canbe an important factor when utilizing such modified electrocatalystproducts in a modified carbon ink, as described in further detail below.Moreover, many of the attributes of electrocatalyst materials are alsodesired attributes of modified electrocatalyst products and any of theabove-described attributes of electrocatalyst materials can beacknowledged as being useful in the production, use and application ofmodified electrocatalyst products. For example, surface area, averageactive species cluster size and size distribution, mass ratio of activespecies phase to carbon support phase, and particle aggregation areimportant factors in catalytic activity. Other attributes are describedbelow.

The modified electrocatalyst product may include varying concentrationsof functional groups, such as from about 0.1 μmol/m² to about 6.0μmol/m². In a preferred embodiment, the modified electrocatalyst producthas a surface group concentration of from about 1.0 μmol/m² to about 4.5μmol/m², and more preferably of from about 1.5 μmol/m² to about 3.0μmol/m².

The modified electrocatalyst product may also have more than onefunctional group and/or linking group attached to the carbon material.In one aspect of the present invention, the modified electrocatalystproduct includes a second organic surface group having a secondfunctional group (Y′) attached to the carbon support. In one embodiment,the second functional group (Y′) can be attached to the carbon supportvia a first linking group (R), which also has the first functional group(Y) attached thereto. Alternatively, the second functional group (Y′)can be attached to the carbon support phase via a separate secondlinking group (R′). In this regard, any of the above referenced organicgroups can be attached as the first and/or second organic surfacegroups, and in any combination.

In a particular embodiment, the first organic surface group includes afirst proton conductive functional group, such as a sulfuric and/orcarboxylic group, and the second organic surface group includes a secondproton conductive functional group, such as a phosphoric group. The useof two different proton conducting functional groups on the same carbonmaterial is useful in circumstances where a wide range of operatingconditions may be utilized so one of the proton conducting groups isalways functional. This enables a relatively flat rate of protonconduction over a wide range of operating conditions. For example,sulfuric groups are known to fail at temperatures of about 100° C.However, phosphoric groups are capable of conducting protons attemperatures above 100° C. Thus, utilizing a modified carbon producthaving two different proton conducting functional groups can enableproton conduction over a wide range of temperatures without requiringthe incorporation of numerous conventional proton conducting materialsin the fuel cell component. Such materials are especially useful in fuelcells utilized in automobiles and other transportation devices wheretemperatures can widely vary during start-up conditions.

Methods of producing modified electrocatalyst products are described infurther detail below. It will be appreciated that many of such methodscan be utilized to produce a modified electrocatalyst product havingfirst and second organic surface groups attached thereto. Preferredmethods for producing modified electrocatalyst products having twodifferent types of organic surface group attached thereto (amultiply-modified electrocatalyst product) include spray processing andsurface contacting techniques, such as immersion and spraying.

In one embodiment, a multiply-modified electrocatalyst product havingfirst and second organic surface groups is produced by spray processing,where a diazonium salt and modified electrocatalyst product having afirst organic surface group attached thereto are included in a precursorcomposition. The precursor composition is subsequently spray processedto attach a second organic group to the carbon support to produce themultiply-modified electrocatalyst product. The multiply-modifiedelectrocatalyst product may then be utilized in the production of a fuelcell component.

In another embodiment, a multiply-modified electrocatalyst producthaving first and second organic surface groups is produced by placing amodified electrocatalyst product having a first organic surface groupattached thereto in a solution comprising a diazonium salt having asecond organic group. The second organic group from the diazonium saltwill attach to the carbon support to create the multiply-modifiedelectrocatalyst product. The multiply-modified electrocatalyst productmay then be utilized in the production of a fuel cell component.

It will be appreciated, that the multiply-modified electrocatalystproduct can be formed by modifying with the second organic surface groupbefore or after the multiply-modified electrocatalyst product isincorporated into a component of the fuel cell. For example, a modifiedelectrocatalyst product can be utilized in the production of a fuel cellcomponent. Subsequently, the modified electrocatalyst product can becontacted by a diazonium salt to attach the second organic surfacegroup.

In a particular embodiment, a modified electrocatalyst product can beutilized in the production of an electrode. Subsequently, the electrodecan be contacted with a second diazonium salt, such as by immersionand/or spraying, to attach the second organic surface group.

Manufacture of Modified Electrocatalyst Products

Modified electrocatalyst products according to the present invention canbe manufactured by any appropriate method, including Impregnation,co-precipitation and other methods utilized by those skilled in the artto make supported electrocatalysts. One preferred method formanufacturing modified electrocatalyst products is spray processing, asdescribed above in reference to modified carbon particles.

When a non-modified carbonaceous material is utilized in a sprayprocessing precursor composition, a dispersant, such as a surfactant, istypically required to enable dispersion and increased loading of thecarbonaceous material. Such dispersants typically require hightemperature processing to facilitate their removal from the resultantproducts. Moreover, in the production of electrocatalyst materials, anyunremoved dispersants typically poison the active sites.

However, according to the present invention, modified carbon productshaving surface groups that match the polar or non-polar nature of theprecursor liquid composition can be used. Such modified carbon productsdecrease or eliminate the need for such dispersants as the modifiedcarbon products may be more readily dispersed in the precursorcomposition. Utilizing modified carbon products may also lower sprayprocessing manufacturing temperatures. Processing at a lower temperaturealso enables reduction of the active species crystallite size in theelectrocatalyst.

As schematically depicted in FIG. 6, and with specific reference toplatinum as the active species phase and carbon black as the carbonmaterial, as processing temperature increases, as depicted left to rightin the figure, crystallite size increases. Conversely, as temperaturedecreases, crystallite size also decreases. Reduced crystallite sizes atlower temperatures are also evidenced due to the decreased ability forthe active species phase (e.g., platinum) to migrate during theproduction temperature.

An increased dispersability of the carbon material in the precursorcomposition also enables an expanded range of carbon products (e.g.,graphite) and metal precursors that can be used. Other materials thatmay be added to the precursor composition include those that do notdecompose during processing, such as ionomers (e.g., PTFE) and molecularspecies (e.g., metal porphryns).

Thus, one approach of the present invention is directed to theproduction of modified electrocatalyst particles by spray processingutilizing a modified carbon product in the precursor composition.According to one particular aspect, modified electrocatalyst productsare produced utilizing spray processing, where the precursor compositionincludes modified carbon product particles as the support phase and aprecursor to the active species.

Preferred modified carbon products useful in accordance with this aspectinclude those that are miscible in an aqueous precursor composition,including those having polar surface groups, such as those terminatingin hydrophilic and/or proton conducting functional groups as listed inTables I and II, above. Preferred modified carbon products useful inaccordance with the present aspect also include those that are misciblein a non-aqueous precursor composition, including those having non-polarsurface groups, such those terminating in hydrophobic functional groupsas those listed in Table II above. Preferred modified carbon productsuseful in accordance with this aspect also include those that arereadily atomizable to produce an aerosol comprising the modified carbonproduct.

In a particularly preferred embodiment, the modified carbon product usedin the precursor composition is a low-conductivity carbon material(e.g., a carbon black) having a hydrophilic surface group (e.g., asulfuric terminating functional group). In a particular embodiment, theprecursor solution includes from about 5 weight percent to about 15weight percent of the modified carbon product.

Preferred active species precursors include those listed above for theproduction of non-modified electrocatalyst materials, includingmolecular metal precursor compounds, such as organometallics (moleculeswith carbon-metal bonds), metal organics (molecules containing organicligands with metal bonds to other types of elements such as oxygen,nitrogen or sulfur) and inorganic compounds such as metal nitrates,metal halides and other metal salts. The molecular metal precursors canbe either soluble or insoluble in the precursor composition.

Particularly preferred molecular metal precursor compounds are metalprecursor compounds containing nickel, platinum, ruthenium, cobalt andchromium. In one preferred embodiment of the present invention, themolecular metal precursor compound comprises platinum.

Various molecular metal precursors can be used for platinum metal, suchas those described above with reference to the molecular metalprecursors utilized in the production of electrocatalyst materials. Anyknown molecular metal precursors can also be utilized for other metals,including molecular metal precursors of ruthenium, nickel, cobalt andchromium. Preferred precursors for ruthenium include ruthenium (III)nitrosyl nitrate [Ru(NO)NO₃)] and ruthenium chloride hydrate. Onepreferred precursor for nickel is nickel nitrate [(Ni(NO₃)₂]. Onepreferred precursor for cobalt is cobalt nitrate [Co(NO₃)₂]. Onepreferred precursor for chromium is chromium nitrate [Cr(NO₃)₃].

In accordance with this aspect, low temperature spray processingconditions can be utilized to produce a modified electrocatalystproduct. The processing temperature within the spray processor ispreferably less than about 500° C., more preferably les than 400° C.,and even more preferably less than 300° C., The residence time withinthe spray processor is preferably less than about 10 seconds, morepreferably less than 5 seconds, and even more preferably less than 3seconds.

In another embodiment, the precursor composition comprises previouslymanufactured electrocatalyst materials, such as any of those describedabove, and a diazonium salt or a diazonium salt precursor. Theabove-described spray processing methods can be utilized to produce amodified electrocatalyst product based on this precursor composition.

In another aspect of the present invention, spray generation methods areutilized in conjunction with a precursor composition including amodified carbon product to generate an aerosol for use in the sprayprocessing methods. As noted above, dispersants, such as surfactants,have previously been utilized to enable spray processing of non-modifiedcarbon material. Surfactants increase the viscosity and change thesurface tension, disallowing the use of certain generation methods likeultrasonic nebulization. In one embodiment, a precursor compositionincluding modified carbon products and/or modified electrocatalystproducts is utilized in a spray processing method to produce a modifiedcarbon product, wherein the precursor composition is atomized utilizingultrasonic generation, a vibrating orifice or spray nozzles.

Formation of Alloyed, Mixed Metal and/or Metal Oxide ModifiedElectrocatalyst Products

Another aspect of the present invention is directed to the use ofmodified carbon products to produce modified electrocatalyst particlesincluding alloys or mixed metal/metal oxides as the active species.Traditionally, platinum is alloyed with various elements such asruthenium, nickel, cobalt or chromium.

Typically, alloys are produced by the deposition of two or more metalsor metal oxides on the surface of the carbon. Subsequently, themetals/metal oxides are subjected to a high temperature post-processingstep in a reducing atmosphere to alloy the active species. Thispost-processing step reduces any metal oxides and allows the differentspecies to migrate over the surface and coalesce to form an alloy.

According to one embodiment of the present invention, spray processingtechniques are used to produce modified electrocatalyst products havinga platinum alloy having a small metal and/or metal oxide crystallitesize. Small crystallite sizes are possible because the metal can bebound to a surface group (e.g., such as an electron donating carboxylicand/or amine functional group) and steric hindrance of the surface groupprevents large crystallite growth during consecutive steps of metaldeposition and/or post-processing or reduction steps. Other electrondonating surface groups useful in accordance with the present embodimentinclude alcohols, ethers, polyalcohols, unsaturated alkyls or aryls,thiols and amines.

By way of illustration, a carbon material, such as carbon black, can beco-modified with carboxylic and amine groups (e.g., (C₆H₄)CO₂H and(C₆H₄)CH₂NH₂). When this carbon material is treated with a metal salt(e.g., RuCl₃), the metal center will bind to the amine functionality onthe carbon surface. When this material is subsequently exposed to asecond metal salt (e.g. Pt(NH₃)₄(OH)₂ or Pt(NH₃)₄(NO₃)₂), it will weaklybind to the carboxylic group of the carbon surface. When this resultingmultiply-modified carbon product is heated under mild reducingconditions, a finely dispersed alloy (e.g., a platinum plus ruthenium)electrocatalyst is produced.

Mixing of the alloy constituents (e.g., Pt and Ru) at the atomic levelreduces the severity of, or eliminates, the post-processing conditionsthat are required for alloy formation, which in turn reduces crystallitesintering and improves alloy crystallite homogeneity. In one embodiment,the modified electrocatalyst products including a metal alloy areproduced at temperatures not greater than 400° C. During processing, itis preferable to have chemically inert surface groups, such as C₅H₄N,(C₆H₄)NH₂, C₆H₅, C₁₀H₇ or (C₆H₄)CF₃, due to pH effects with theprecursor salts. Additionally, if both metals have the same affinity forthe attached surface group(s), the addition of a homogeneousdistribution of a different metal species leads to an even distributionover the carbon surface. Again, such an approach will result in anatomic distribution of the metal species, for example, Pt and Ru,anchored and distributed evenly over the carbon surface. This evendistribution enables alloy formation at lower processing temperatures,and, hence, smaller alloy crystallite sizes and better catalyticperformance.

According to another embodiment of the present invention, one or moremetal or metal oxide precursors can be deposited on the modified carbonproduct where a post-processing step occurs between the two depositionsteps. For example, a metal or metal oxide precursor can be added to amodified carbon support where a surface of the carbon that is notmodified preferentially adsorbs the precursor, and after apost-processing decomposition step, first metal or metal oxide clustersare formed. This is followed by deposition of a second precursor, whichpreferentially adsorbs on the surface of previously formed first metalor metal oxide clusters. After the second decomposition step, a finecomposite cluster is formed (e.g., where the second element depositseither as a monolayer or as clusters onto the surface of the first metalclusters).

By way of example, and as depicted in FIG. 7, a modified carbon producthaving surface groups relatively inert to an active species phase (e.g.,SiMe₃) can be utilized to selectively produce an alloyed modifiedelectrocatalyst product. A first metal/metal oxide (e.g., RuO₂) can bedeposited on the carbon material surface in locations where the surfacegroups are not located. After a post-processing step, a secondmetal/metal oxide (e.g., Pt) can then be selectively deposited, wherethe second metal/metal oxide preferentially is adsorbed onto the surfaceof the first metal/metal oxide clusters to form a composite activespecies phase cluster and/or a thin surface active layer, which may beused as a support for other deposited materials.

It will be appreciated that one or more of the metal or metal oxidespecies described above can be deposited by any known method, includingliquid phase adsorption (as discussed above), spray-based processing,chemical vapor deposition, under potential deposition, electrolessdeposition or liquid-phase precipitation. In such cases, the carbon canbe modified such that the metal/metal oxide precursors in the seconddeposition process cannot absorb onto anything other than the firstmetal/metal oxide initially deposited, resulting in improved crystallitedispersion, uniformity of alloy formation and elimination of segregationphenomena.

As described above, alloy electrocatalysts typically requirepost-processing to ensure a proper degree of alloying to providelong-term stability of the active sites. During this post-processingstep, the alloy crystallites have a tendency to sinter, resulting incrystallite growth. Modified carbon products can be used to minimizethis effect. With the integration of modified carbons as the supportstructure, the metal/metal oxide dispersion is significantly increasedsince the surface modification blocks part of the surface and inhibitsmigration. This prevents surface diffusion and agglomeration of thealloy clusters during the reduction/alloying step.

For example, modifying the surface of the carbon material with thermallystable steric groups, such as phenyl (C₆H₅) or napthyl (C₁₀H₇) groupsacts to physically block migration of metal and metal oxide speciesacross the surface of the carbon. When an electrocatalyst is producedwith a greater metal/metal oxide dispersion (i.e., smaller crystallitesize), and the species to be alloyed (e.g. mixed metal/metal oxides)requires a lower temperature for alloy formation, a reduced grain growthand better dispersion of the alloy clusters results. Reduced graingrowth leads to smaller alloy crystallite size, increased catalyticactivity and increased precious metal utilization. Additionally, asdescribed previously, where surface modification groups are intact afterthe post-processing procedure, they can sterically prevent the metalcrystallites from growing by blocking diffusion paths.

Modified Carbon Products and Proton Exchange Membranes

As noted above, modified carbon products can be utilized in protonexchange membranes. The use of modified carbon products and/or diazoniumsalts in conjunction with a proton exchange membrane are discussedbelow. It will be appreciated that the various aspects, approaches,and/or embodiments of the present invention described below, areprimarily in reference to modified carbon products in proton exchangemembranes. However, it will be appreciated that electrocatalystmaterials can be utilized in conjunction with modified carbon productsin many of such aspects, approaches and/or embodiments, whereappropriate, although not specifically mentioned, and the use of suchelectrocatalyst materials in such aspects, approaches and/or embodimentsis expressly within the scope and spirit of the present invention.

Proton Conductivity

In a traditional PEM, protons conduct through a membrane, such as asulfonated polytetrafluoroethylene (sulfonated PTFE) membrane, by meansof protonated water (H₃O⁺) that is hydrogen bonded to the sulfonic acidgroups, which in turn are attached to the polymer backbone, asillustrated in FIG. 8. These PEMs generally require the presence ofwater for efficient proton conductivity.

According to one embodiment of the present invention, a modified carbonproduct is utilized to increase the amount of proton conducting siteswithin the PEM. Increasing the concentration of proton conducting sitesis beneficial for several reasons, including increased transport ofprotons across the membrane and a decrease in the amount of water neededto be supplied with the anode reactant. In some situations, eliminationof the water supplied with the anode reactant can be achieved, whichgreatly reduces the complexity and design of the fuel cell. An addedbenefit is that surface groups having proton conducting functionalgroups are also generally hydrophilic. Thus, increasing the amount ofproton conducting material also increases the water retention capabilityof the fuel cell, thereby enabling rapid “dry” starts.

Similarly, the functional group can be tailored to increase K_(a)(proton donating strength, like K_(a): —SO₃H>—CO₂H), to form a betterproton conductor. In addition, a tailored functional group may also beutilized to introduce different chemical reactivity to the membrane,such as by increasing hydrogen bond strength, which may affect thebonding strength of a modified carbon product to the membrane surface.

The linking group can also vary. Particularly preferred branched andunbranched linking groups are listed in Table V. It will be appreciatedthat any proton conducting functional group (Y) can be utilized inconjunction with any linking group (R) to create a modified carbonproduct in accordance with the present invention. In a particularapproach, the linking group is tailored to increase the “reach” of theproton-conducting functional group by adding flexibility and degrees offreedom and further increase proton conduction. According to onepreferred embodiment, the linking group is selected such that thefunctional group extends to, but not substantially beyond, the adjacentfunctional group.

One example of increased proton conductivity utilizing tailored linkinggroups is the use of a modified carbon black having (C₆H₄)(CH₂)₅SO₃Hfunctional groups. This modified carbon product can be incorporated intoa membrane to provide a longer effective “reach” and increasedflexibility of the proton conducting group. In the case of a(C₆H₄)(CH₂)₅SO₃H modified carbon product, the saturated (CH₂)₅ chainattached to the phenyl ring adds significant length and an increaseddegree of rotational flexibility about several C—C single bonds,resulting in an increased cone angle. In this situation, the aliphaticchain allows for increased proton conductivity, especially in dry,reduced humidity or non-humidified conditions.

Modified carbon products can be utilized in conjunction with any of avariety of types of PEMs. These include fluorinated PEMs such assulfonated PTFE, perfluorosulfonated PTFE, polyvinylidene fluoride(PVDF), as well as acid-doped or derivatized hydrocarbon polymers, suchas polybenzimidizole (PBI), polyarylenes, polyetherketones,polysulfones, phosphazenes and polyimides and other similar membranes. Aschematic illustration of a resulting structure is illustrated in FIG.9. As illustrated in FIG. 9, a RSO₃H(R=linking group) modified carbonproduct is added to a sulfonated PTFE membrane, which results in anincreased concentration of proton conductive groups.

PBI is a proton conducting membrane that is often used inhigh-temperature PEMFCs, and generally includes hydrogen-bonded H₃PO₄species in high concentrations. H₃PO₄ is typically lost during fuel celloperation as it migrates toward the membrane surface due toelectro-osmotic drag, where it subsequently evaporates. In oneembodiment of the present invention, the proton conducting modifiedcarbon products, such as those having proton conducting surface groups(e.g., those containing sulfuric and/or phosphonic functional groups),can be incorporated into a PBI membrane to increase the concentration ofthe proton conducting sites, and hence proton conductivity of the PBImembrane.

In a particularly preferred embodiment, modified carbon products havingphosphonic groups are incorporated into a PBI membrane, as isillustrated in FIG. 10. Adding modified carbon products havingphosphonic functional groups to PBI-based membranes increases the numberof proton conducting sites and increases the hydrogen bonding to theH₃PO₄ acid bound within the pores of the membrane. This prevents the PBImembrane from losing H₃PO₄ acid over time.

One example of increased proton conductivity due to an increased protonconducting material density in a proton conducting membrane is the useof modified carbon black having (C₆H₄)SO₃H surface groups. Preferably,the concentration of surface groups on the modified carbon black is atleast about 3.0 μmol/m², and more preferably at least about 5.0 μmol/m².Preferably, the volume density of proton conducting groups on themodified carbon black is at least about 5.0 mmol/mL, more preferably atleast about 5.4 mmol/mL. This modified carbon black, when mixed into aperfluorintated sulfonic acid (PFSA)(e.g., NAFION available from E.I.duPont de Nemours, Wilmington, Del.) solution (e.g., at a 50:50 volumeratio), can be utilized to produce a modified proton exchange membrane.In one embodiment, the volume density of proton conducting groups on theresulting membrane is at least about 5 mmol/mL (about 2.7 mmol/g).

Proton conducting modified carbon products can also be advantageouslyincorporated into non-proton conducting membranes, such as nonacid-doped or derivatized PBI, polyimides, polysulfones,polyphosphazenes, polyetherketones (PEEKs) and polysiloxanes. Despitelacking proton conductivity, such membranes can provide chemicalrobustness, mechanical strength and good electrical insulation. Bymodification of such membranes with proton conducting modified carbonproducts, a modified membrane can be formed having proton conductivity.This is advantageous in cases where a certain set of physical and/orchemical properties are desired that can only be achieved through theuse of a non-proton conducting “base” membrane. Furthermore,incorporating modified carbon products into a non-proton conductingmembrane enables the disconnection of the proton conductivity of thefinal membrane from the membrane material utilized during fabrication.

Mechanical Strength Enhancement

Modified carbon products can also be utilized according to the presentinvention to increase the mechanical strength of the PEM. In oneembodiment of the present invention, a modified carbon product having apolymeric functional group is utilized to create physical and/orchemical interactions with the membrane. For example, a modified carbonhaving a surface group with aliphatic surface groups can physicallyinteract (e.g., intertwine) with the polymer membrane backbone toincrease the strength of the membrane. Surface groups useful inaccordance with the present invention include acrylics,polypropyleneoxide (PPO), polyethyleneoxide (PEO) polyethylene glycol(PEG) and polystyrene. In a particularly preferred embodiment, theconcentration of surface groups on the carbon material is from about 0.1to about 5 μmoles/m².

According to another embodiment of the present invention, the modifiedcarbon product has at least one surface group that forms a chemical bondto the sulfonate proton exchange membrane. For example, a modifiedcarbon product having an amine-containing (NH₂) surface group can beincorporated into a proton exchange membrane, such as a PTFE membrane.The NH₂ groups hydrogen bond to the oxygen-containing sulfonate groups,resulting in a membrane having increased strength. Other surface groupsuseful in accordance with the present embodiment include SO₃H, PO₃H₂ andCO₂H groups.

In another embodiment, modified carbon products having a surfacefunctional group can be incorporated in the raw polymer material priorto forming the membrane to increase mechanical strength of the membrane.For example, a modified carbon product having an amine surface group canbe added to a carboxylate-based PTFE membrane and processed to create amechanically enhanced proton exchange membrane. In this regard, the NH₂groups react with the carboxylate groups on the polymer during polymerformation, forming an amide of the form RC(O)N(H)R and water.

In another embodiment, the surface groups on the carbon materials can beselected to covalently bond to the polymer backbone through a number ofmeans, such as cross-polymerization or condensation. For example, anethylene-terminated modified carbon product can be added to a batch ofperfluorosulfonated PTFE comonomer prior to final membrane formation.Polymerization is initiated and the modified carbon product (through theethylene surface functional group) co-polymerizes with the co-monomersresulting in a modified composite membrane including carbon covalentlybound to the polymer.

It will be appreciated that the foregoing embodiments (e.g., increasedproton conductivity and increased mechanical strength) are not mutuallyexclusive. The modified carbon product can advantageously increase themechanical strength while the resulting proton conductivity remainsunchanged. Furthermore, the addition of proton conducting modifiedcarbon products to the membrane can increase both the mechanicalstrength and the proton conductivity of the membrane. The modifiedcarbon products can be co-modified with a long chain functional groupand a proton-conducting group. That is, the carbon particles can includemore than one type of surface modification. For example, carbon black(e.g., VULCAN XC-72, Cabot Corp., Boston Mass.) can be modified with 1μmol/m² of PEG and also with 3 μmol/m² of (C₆H₄)SO₃H to produce amultiply-modified carbon product. When this multiply-modified carbonproduct is introduced into a substantially non-proton conductingmembrane, such as polyimide, the resultant membrane will evidenceenhanced proton conductivity and increased mechanical strength. It willbe appreciated that two different modified carbons can be utilized toderive the benefit of two different functional groups. For example, afirst modified carbon product having a polymeric functional group (e.g.,PEG) and a second modified carbon product having a proton conductingfunctional group (e.g., (C₆H₄)SO₃H) can be utilized in the production ofa proton exchange membrane to produce a membrane with enhanced protonconductivity and mechanical strength.

The thermal stability of the membrane can also be enhanced according tothe present invention. Carbon is very thermally stable compared toorganic polymers, and increasing the loading of carbon within the PEMwill increase the overall thermal stability of the membrane. A thermallystable PEM can be produced by the addition of a thermally stablemodified carbon to a standard PEM. Moreover, the PEM can be reinforcedwith the addition of acicular, rod or whisker like modified carbonproducts. Alternatively, thermally stable PEM can be produced by theaddition of a proton conducting modified carbon product to a non-protonconducting but thermally stable membrane such as, for example,polyimide. In either case, the blending of two or more constituents,such as a membrane and a modified carbon product or a membrane and twodifferent modified carbon products or a carbon material modified withmore than one surface group, can result in a unique combination ofproperties.

Water and Fuel Transport Control, Humidification and Porosity

Water transport is a key issue for PEMs. One feature that the modifiedPEMs according to the present invention can impart to the MEA structureis the ability to operate at decreased humidification levels due to anincreased concentration of proton conducting groups per unit volume. Inthis regard, one aspect of the present invention relates to modifiedcarbon products including proton conductive functional groups. Thisleads to a significant overall increase in the concentration of protonconducting sites per unit volume, and as well as increased mobility(reach), which allows protons to migrate through the membrane withoutwater acting as a bridge. Moreover, the proton exchange membrane mayalso operate at a significantly lower humidification level. The abilityto operate a proton exchange membrane in near humid and/or dryconditions greatly simplifies the fuel cell design, especially at highoperating temperatures, where the difficulty associated with addingwater to the membrane is heightened by the accelerated evaporation rateof the water.

The integration of modified carbon products can significantly affect thehumidification requirements and water transport for fuel cell systemsutilizing PEMs. In a particular embodiment, a composite modified carbonproduct/polymer membrane is utilized to increase proton conduction atreduced humidification levels. Moreover, even where the modifiedmembranes do not have a high enough concentration of proton conductiongroups to allow fully dry conduction, the addition of stronglyhydrophilic groups, such as SO₃H, advantageously decreases the requisitewater vapor pressure, allowing for water-based proton transfer atelevated temperatures and/or reduced humidification levels of thereactant gases.

According to another embodiment of the present invention, the averagepore size within the polymer membranes is reduced through the use ofmodified carbon products having polymeric surface groups, as describedabove, to decrease permeability and, therefore, likelihood of water orfuel transport through the membrane. For example, the use of a modifiedcarbon product having hydrophilic and long chained functional groupsattached thereto increases proton conductivity while also forming adenser membrane with a smaller average internal pore size.

The porosity of the PEM can be tailored by selecting appropriate carbonmaterials, linking groups and functional groups. Porosity can betailored to prevent and/or inhibit gas, water and/or fuel transportacross the membrane. In one embodiment, porosity is tailored byutilizing a high surface area carbon black having a high density ofproton conducting groups co-modified with large polymeric groups, suchas PEG or PPO having a relatively low electrical conductivity. One suchcarbon black is KETJEN BLACK (available from Akzo Nobel), which has asurface area of about 1600 m²/g and an electrical resistivity of about500 to 1500 milliOhm-cm. In a particular embodiment, the carbon blackcan be modified with a proton conducting group, such as a sulfonate, andlong-chained polymeric group, such as PEG, and formulated into amembrane having a small pore size. In one preferred embodiment, themembrane has an average pore size of no greater than 3 angstroms.

As will be appreciated, the small pore size is achieved due to thepolymeric groups (e.g., PEG) having significant steric bulk danglingfrom the carbon surfaces. To enhance/tailor porosity, it is important toutilize such large molecular groups, like PEG or PPO, to act as a binderand “filler” between the carbon materials. Without the addition of suchsteric inhibitors, it is difficult to achieve a dense membrane suitablefor use in a fuel cell. The length of the linking group regarding theproton conducting functionality should be such that it extends to, butnot substantially beyond, the next proton-conducting functional group.In one extreme, the resulting membrane is essentially based uponmodified carbon products and is proton conducting.

One embodiment of such an approach utilizing sulfonic acid as the protonconducting functional group is depicted schematically illustrated inFIG. 11. Modified carbon products 1124 are protonically connected viathe sulfonic acid functional groups 1150 extended by the linking groups(R), resulting in a membrane with increased proton conductivity anddecreased porosity.

Direct methanol fuel cells (DMFCs), as well as fuel cells that utilizeother hydrogen-containing liquid fuels, like ethanol, also utilize aproton conductive membrane. In a DMFC, methanol migration through themembrane (i.e., methanol cross-over) should be avoided to maintain fuelcell performance. Methanol cross-over results in both low fuelutilization and poisoning of active catalyst sites at the cathode, whichsignificantly decreases the efficiency and performance of the fuel cell.

According to the present invention, methanol cross-over through themembrane can be reduced by the addition of modified carbon products tothe membrane. As with PEMFCs, increasing the density of protonconduction sites will allow protons to be transported without the needfor water. In addition, decreasing the porosity utilizing modifiedcarbon products having a polymeric surface group, as described above,can also decrease methanol cross-over.

For purposes of illustration, a carbon material, such as carbon black,can be modified with a proton conducting functional group and afunctional and/or linking group having substantial steric bulk (e.g.,PEG). This modified carbon product can be mixed with another modifiedcarbon product having proton conducting functional groups (e.g.,(C₆H₄)SO₃H). This mixture can be added to a perflurosulfonated PTFEpolymer co-monomer, and cast to form a modified membrane having anincreased proton conductivity and reduced porosity, resulting in lowermethanol cross-over levels. Another embodiment of the present inventionutilizes modified carbon products to tailor thehydrophilicity/hydrophobicity of the internal pores of the membrane toselectively transport water and inhibit the transport of othermolecules, such as methanol.

Fabrication of PEMs Utilizing Modified Carbon Products

There are a variety of approaches to producing PEMs including modifiedcarbon products, which are discussed in further detail below. It will beappreciated that any of the above-described modified carbon products,including modified electrocatalyst products, and/or electrocatalystmaterials may be utilized in any of the below-described approaches,aspects and/or embodiments to produce and/or modify a proton exchangemembrane. It will also be appreciated that the thickness of the membraneshould be suitable for the specific fuel cell application, irrespectiveof its manufacturing process. Preferably, the thickness of the resultantproton exchange membrane is from about 25 to about 250 microns, morepreferably from about 30 microns to about 150 microns, and even morepreferably from about 35 microns to about 75 microns.

In one approach, a pre-existing PEM may be contacted with a diazoniumsalt to attach an organic group to carbon materials contained therein tocreate a PEM including a modified carbon product. It will be appreciatedthat any of the below referenced deposition methods can be utilized todirectly deposit a diazonium salt onto a carbonaceous surface for thepurpose of directly modifying such carbonaceous surface. Additionally,such deposition techniques can be used to modify any carbonaceousmaterials contained in the PEM, including non-modified carbon materials,electrocatalyst materials, modified carbon products and/or modifiedelectrocatalyst products. It will also be appreciated that suchdeposition techniques can be utilized to create a uniform modifiedcarbon layer across the entire surface of the proton exchange membrane,or can be deposited in discrete patterns to produce patterned modifiedcarbon layers.

In another approach, modified carbon products may be utilized in theextrusion and casting process to create a modified carbon PEM. In oneaspect, a modified carbon product is mixed into a monomer prior topolymerization and casting/extrusion to form the modified carbon PEM. Inanother aspect, the modified carbon product is mixed into the polymerprior to casting/extrusion to form the PEM. In one embodiment, amodified carbon product is included in the fluid (e.g., monomer orpolymer-containing fluid) utilized in the casting and extrusion processto create a modified carbon proton exchange membrane. In a particularembodiment, the modified carbon product is tailored to increases itssolubility and/or dispersability in the fluid, such as by utilization ofspecific functional groups. Such increased solubility and/ordispersability within the fluid enables formation of a composite PEMwith uniform distribution of the modified carbon product. In aparticular embodiment, the surface group of the modified carbon productis both hydrophilic and proton conducting.

By way of illustration, a modified carbon product including a protonconducting functional group (e.g., (C₆H₄)SO₃H attached to carbon black)can be added to a formulation including a PFSA. This formulation can befabricated into a membrane by casting or extrusion. In one embodiment,the formulation includes various amounts of modified carbon products,such as at least about 20 wt. %, more preferably at least about 40 wt. %and up to about 70 wt. %. After fabrication, the PEM may include from 5wt. % to 30 wt. % of a modified carbon product. The resulting membrane,after casting or extrusion, evidences an increased proton conductivityat, or even below, typical PTFE-type humidification levels. In addition,such membranes may also evidence an increased mechanical stability,depending on the amount and type of modified carbon product utilized.

In another example, modified carbon products can be mixed with anon-proton conducting membrane material to form a composite mixture.Preferably, the modified carbon product constitutes at least about 10vol. % of the dry mixture, up to about 70 vol. % of the dry mixture. Thedry mixture can be added to a solvent and blended, if necessary, such asby using a ball mill. Solvent systems can include aqueous solvents,non-aqueous solvents and mixtures thereof. The resulting slurry can becast and dried to form a membrane. The dried and cast membrane can bepressed (to further reduce thickness) and/or can be heated to anelevated temperature, such as near or slightly above the glasstransition temperature (T_(g)) of the polymer.

In another approach, modified carbon products may be deposited onto orimpregnated into a pre-existing PEM. In one aspect, the modified carbonproducts may be deposited or impregnated using a modified carbon ink, asdiscussed in further detail below. In one embodiment, the protonexchange membrane may be contacted by a modified carbon ink, such as byimmersion or spraying, to impregnate and/or deposit the modified carbonproduct into/onto the membrane. In a particular embodiment, the modifiedcarbon ink is deposited utilizing a direct-write tool. In anotherembodiment, the modified carbon ink is sprayed onto a membrane (e.g., apolybenzimidazole membrane) to form a coated membrane. In particularembodiment, the coated membrane can be further contacted with a solutioncontaining additional materials, such as a proton conducting material orpolymer backbone material. In a particular embodiment a coatedpolybenzimidazole membrane is immersed in a phosphoric acid (H₃PO₄)solution to increase the amount of proton conducting groups in themembrane.

In yet another approach, proton-conducting membranes may be fabricateddirectly from one or more modified carbon products. This approachalleviates the need for conventional polymers, membrane materials and/orbinder systems, which eliminates the negative effects of such materialson the PEM. In this regard, it will be appreciated that large chain,polymeric insulating groups can be attached to the carbon materials,essentially coating them and making them electrically insulating, whichaids in the production of PEMs without the above described conventionalpolymers, membrane materials and/or binder systems. Moreover, byselecting the proper combination of proton conducting functional group,linking group and carbon, a PEM having the desired properties can beobtained. In one embodiment, the PEM is fabricated by deposition of amodified carbon ink, as is discussed in further detail below.

As noted above, various methods may be utilized to incorporate amodified carbon product in a proton exchange membrane. One particularmethod includes the steps of contacting a carbon material with adiazonium salt to form a modified carbon product and incorporating themodified carbon product into the proton exchange membrane. It will beappreciated that more than one type of carbon material and/or diazoniumsalt may be utilized in this approach to form a plurality of modifiedcarbon products and/or multiply-modified carbon products.

One specific embodiment utilizes spray processing techniques, andincludes the steps of providing a precursor composition including acarbon material and a diazonium salt, spray processing the precursorcomposition to form a modified carbon product, and incorporating themodified carbon product into a proton exchange membrane.

Another specific embodiment includes the steps of providing a precursorcomposition including an electrocatalyst material and a diazonium salt,spray processing the precursor composition to form a modifiedelectrocatalyst product, and incorporating the modified electrocatalystproduct into a proton exchange membrane.

Yet another specific embodiment includes the steps of providing aprecursor composition including an existing modified carbon product andan active species precursor, spray processing the precursor compositionto form a modified electrocatalyst product, and incorporating themodified electrocatalyst product into a proton exchange membrane.

Another method for incorporating a modified carbon product in a protonexchange membrane, includes the steps of mixing a modified carbonproduct with another material (e.g., a second modified carbon product, aconventional polymer used in the production of a PEM, an electrocatalystmaterial, a conventional carbon material, a resin and/or other materialsutilized in the production of a proton exchange membrane) to form amodified carbon-containing mixture and incorporating the mixture intothe proton exchange membrane. It will be appreciated that more than onetype of modified carbon product and other material may be utilized inthis approach to form the mixture.

In one specific embodiment, modified carbon products are dispersed in anink to create a modified carbon ink that may be utilized in theproduction of a proton exchange membrane, such as by analog or digitalprinting, as discussed in further detail below.

Yet another method includes the steps of incorporating a carbonaceousmaterial, such as a modified carbon product and/or a conventional carbonmaterial into a proton exchange membrane and contacting the carbonaceousmaterial with a diazonium salt to form a modified carbon product in theproton exchange membrane. It will be appreciated that more than one typeof carbonaceous material (e.g., modified carbon product) and/ordiazonium salt may be utilized in this approach to form a plurality ofmodified carbon products and/or multiply-modified carbon products. In aspecific embodiment, a diazonium salt is deposited using a direct-writetool, as discussed in further detail below, to form a modified carbonproduct in the proton exchange membrane.

Interface with Adjacent Components/Adhesion

Another embodiment of the present invention is directed to theincorporation of modified carbon products to improve the interfacecontact between the proton exchange membrane and the electrode layer. Inthis regard, the surface group can form a physical or chemical bondbetween the materials on either the electrode or the proton exchangemembrane due to improved dispersability of the modified carbon product,forming a smoother surface and increasing contact area.

By way of illustration, modification of an electrocatalyst material witha hydrophilic functional group enables preparation of a uniform, lowviscosity aqueous ink, which can be formed into a smooth, crack-freeelectrode layer. Such a layer increases contact because the hydrophilicfunctional groups of the modified electrocatalyst product can hydrogenbond to any adjacent polymer groups of the proton exchange membrane.This improves bonding between the electrode and the proton exchangemembrane, thereby decreasing contact resistance and minimizing ohmiclosses. Another benefit is reduced delamination and structuraldeterioration during long term operation of the fuel cell.

Another embodiment of the present invention is directed to the use ofmodified carbon products to increase the adhesion of between an theproton exchange membrane and adjacent fuel cell component (e.g., theelectrode). A proton exchange membrane incorporating modified carbonproducts may form a physical interlocking bond or a chemical bond to theadjacent component. In this regard, polymeric functional groups and/orlinking groups can be utilized to increase adhesion of the gas/fluiddiffusion. Such polymeric functional groups can be physicallyintertwined with the surface of the adjacent component, which increasesadhesion. In another embodiment, hydrophilic functional groups can beutilized to increase hydrogen bonding between the proton exchangemembrane and the adjacent component. It will be appreciated that bothhydrophilic functional groups and/or polymeric functional and/or linkinggroups can be utilized on a modified carbon product(s) to achieve suchproperties.

Deposition of Modified Carbon Products and/or Diazonium Salts

In one aspect, modified carbon products may be utilized in a modifiedcarbon ink to produce and/or modified the PEM, such as analog printing(screen, lithographic, roll-coat, slot die and flexographic) and digitalprinting (e.g., direct-write, spraying, electrostatic, xerographic,laser transfer) methods as is discussed in further detail below. As usedherein a “modified carbon ink” refers to any liquid phase solution, suchas an ink, resin or paste, that contains one or more of theabove-described modified carbon products and/or modified electrocatalystproducts. As used herein “electrocatalyst inks” refers to a liquid phasesolution, such as an ink, resin or paste, that contains one or more ofthe above-described electrocatalyst materials.

Various aspects, approaches, and/or embodiments of the present inventionare described below, primarily in reference to modified carbon inks.However, it will be appreciated that electrocatalyst inks can beutilized in conjunction with a modified carbon ink in some of suchaspects, approaches and/or embodiments, where appropriate, although notspecifically mentioned, and the use of such electrocatalyst inks in suchaspects, approaches and/or embodiments is expressly within the scope andspirit of the present invention.

The incorporation of modified carbon products in a modified carbon inksignificantly improves ink uniformity, homogeneity, ease of manufactureand ease of use. Various methods and mixing techniques are currentlyutilized to improve the properties of inks that includeelectrocatalysts, carbons and polymer solutions (e.g., PFSA or PTFE) andcombinations thereof, such as ball milling and sonication. Theincorporation of modified carbon products having surface groups thatmatch the solubility requirements of the ink there are dispersed insignificantly simplifies ink preparation As a result, the homogeneityand uniformity of the inks, and hence the homogeneity of the depositedlayer/feature are increased. Homogenous deposition enables control overthe concentration and drying rate of the materials being deposited. Forexample, a modified carbon product having hydrophilic surface groupssimplifies dispersion of carbon-based materials in aqueous-based inksdue to increased wetting and dispersability of the modified carbonmaterial. Other surface modifications can be chosen to improve thewettability and dispersability of modified carbon products when organicsolutions are used.

In one embodiment of the present invention, modified carbon products areutilized in a PFSA solution and/or a PTFE suspension to create amodified carbon ink, where the aggregate size of the modified carbonparticles is not larger than the size of the largest particle within theink.

In a particular embodiment, a modified carbon product having twodifferent surface groups (e.g. a hydrophilic and a hydrophobic group) isutilized in a PFSA solution and/or PTFE suspension to create a modifiedcarbon ink, where the aggregate size of the modified carbon products isnot larger than the size of the largest particle within the ink.

Deposition of modified carbon inks preferably produce and/or modify thePEM to enable proton conductivity, physical separation, and/orelectrical insulation, after deposition and/or post-processing. In thisregard, it should be noted that any combination of surface groupsdescribed in U.S. Pat. No. 5,900,029 by Belmont et al. can be utilizedin conjunction with any modified carbon product in and modified carbonink to create and/or modify the proton exchange membrane. Preferably,the modified carbon ink is formulated for deposition (e.g., via analogor digital printing) to maintain a low manufacturing cost whileretaining the above noted properties.

The modified carbon inks according to the present invention can bedeposited to form patterned or unpatterned layers using a variety oftools and methods. In one embodiment, a modified carbon ink is depositedusing a direct-write deposition tool. As used herein, a direct-writedeposition tool is a device that can deposit a modified carbon ink ontoa surface by ejecting the composition through an orifice toward thesurface without the tool being in direct contact with the surface. Thedirect-write deposition tool is preferably controllable over an x-ygrid. One preferred direct-write deposition tool according to thepresent invention is an ink-jet device. Other examples of direct-writedeposition tools include aerosol jets and automated syringes, such asthe MICROPEN tool, available from Ohmcraft, Inc., of Honeoye Falls, N.Y.

An ink-jet device operates by generating droplets of a liquid suspensionand directing the droplets toward a surface. The position of the ink-jethead is carefully controlled and can be highly automated so thatdiscrete patterns of the modified carbon ink/electrocatalyst ink can beapplied to the surface. Ink-jet printers are capable of printing at arate of 1000 drops per second per jet, or higher, and can print linearfeatures with good resolution at a rate of 10 cm/sec or more, such as upto about 1000 cm/sec. Each drop generated by the ink-jet head includesapproximately 25 to 100 picoliters of the suspension/ink that isdelivered to the surface. For these and other reasons, ink-jet devicesare a highly desirable means for depositing materials onto a surface.

Typically, an ink-jet device includes an ink-jet head with one or moreorifices having a diameter of not greater than about 100 μm, such asfrom about 50 μm to 75 μm. Droplets are generated and are directedthrough the orifice toward the surface being printed. Ink-jet printerstypically utilize a piezoelectric driven system to generate thedroplets, although other variations are also used. Ink-jet devices aredescribed in more detail in, for example, U.S. Pat. No. 4,627,875 byKobayashi et al. and U.S. Pat. No. 5,329,293 by Liker, each of which isincorporated herein by reference in its entirety. Functionalized carbonparticles have been demonstrated to be stable in inks at relatively highcarbon loadings by Belmont et al. in U.S. Pat. No. 5,554,739, which isincorporated herein by reference in its entirety. Ink-jet printing forthe manufacture of DMFCs is disclosed by Hampden-Smith et al. incommonly-owned U.S. patent application Ser. No. 10/417,417 (PublicationNo. 20040038808) which is also incorporated herein by reference in itsentirety.

It is important to simultaneously control the surface tension and theviscosity of the modified carbon ink to enable the use of industrialink-jet devices. Preferably, the surface tension of the ink is fromabout 10 to 50 dynes/cm, such as from about 20 to 40 dynes/cm. For usein an ink-jet, the viscosity of the modified carbon ink is preferablynot greater than about 50 centipoise (cp), such as in the range of fromabout 10 cp to about 40 cp. Automated syringes can use compositionshaving a higher viscosity, such as up to about 5000 cp.

According to one embodiment, the solids loading of modified carbonproducts in the modified carbon ink is preferably as high as possiblewithout adversely affecting the viscosity or other necessary propertiesof the composition. For example, a modified carbon ink can have a solidsloading of up to about 20 wt. %. In one embodiment, the solids loadingis from about 2 wt. % to about 10 wt. %. In another particularembodiment, the solids loading is from about 2 wt. % to about 8 wt %. Asis discussed below, the surface modification of a carbon product canadvantageously enhance the dispersion of the carbon product, and lead tohigher obtainable solids loadings.

The modified carbon inks used in an ink-jet device can also includewater and/or an alcohol. Surfactants can also be used to maintain themodified carbon products in the ink. Co-solvents, also known ashumectants, can be used to prevent the modified carbon inks fromcrusting and clogging the orifice of the ink-jet head. Biocides can alsobe added to prevent bacterial growth over time. Examples of such liquidvehicle compositions for use in an ink-jet are disclosed in U.S. Pat.No. 5,853,470 by Martin et al.; U.S. Pat. No. 5,679,724 by Sacripante etal.; U.S. Pat. No. 5,725,647 by Carlson et al.; U.S. Pat. No. 4,877,451by Winnik et al.; U.S. Pat. No. 5,837,045 by Johnson et al.; and U.S.Pat. No. 5,837,041 by Bean et al. Each of the foregoing U.S. patents ishereby incorporated herein by reference in its entirety. The selectionof such additives is based upon the desired properties of thecomposition. If necessary, modified carbon products can be mixed withthe liquid vehicle using a mill or, for example, an ultrasonicprocessor. In this regard, it should be noted that modified carbonproducts that are dispersible in their corresponding solvent (e.g. amodified carbon product having a hydrophilic surface groups in anaqueous solution) may require minimal or no mixing due to their improveddispersability in their corresponding solvents.

The modified carbon inks according to the present invention can also bedeposited by aerosol jet deposition. Aerosol jet deposition can enablethe formation of features having a feature width of not greater thanabout 200 μm, such as not greater than 100 μm, not greater than 75 μmand even not greater than 50 μm. In aerosol jet deposition, the modifiedcarbon ink is aerosolized into droplets and the droplets are transportedto a substrate in a flow gas through a flow channel. Typically, the flowchannel is straight and relatively short. For use in an aerosol jetdeposition, the viscosity of the ink is preferably not greater thanabout 20 cp.

The aerosol in the aerosol jet can be created using a number ofatomization techniques, such as by ultrasonic atomization, two-fluidspray head, pressure atomizing nozzles and the like. Ultrasonicatomization is preferred for compositions with low viscosities and lowsurface tension. Two-fluid and pressure atomizers are preferred forhigher viscosity inks.

The size of the aerosol droplets can vary depending on the atomizationtechnique. In one embodiment, the average droplet size is not greaterthan about 10 μm, and more preferably is not greater than about 5 μm.Large droplets can be optionally removed from the aerosol, such as bythe use of an impactor.

Low aerosol concentrations require large volumes of flow gas and can bedetrimental to the deposition of fine features. The concentration of theaerosol can optionally be increased, such as by using a virtualimpactor. The concentration of the aerosol can be greater than about 10⁶droplets/cm³, such as greater than about 10⁷ droplets/cm³. Theconcentration of the aerosol can be monitored and the information can beused to maintain the mist concentration within, for example, 10% of thedesired mist concentration over a period of time.

Examples of tools and methods for the deposition of fluids using aerosoljet deposition include U.S. Pat. No. 6,251,488 by Miller et al., U.S.Pat. No. 5,725,672 by Schmitt et al. and U.S. Pat. No. 4,019,188 byHochberg et al. Each of these patents is hereby incorporated herein byreference in its entirety.

The modified carbon inks of the present invention can also be depositedby a variety of other techniques including intaglio, roll printer,spraying, dip coating, spin coating and other techniques that directdiscrete units, continuous jets or continuous sheets of fluid to asurface. Other printing methods include lithographic and gravureprinting.

For example, gravure printing can be used with modified carbon inkshaving a viscosity of up to about 5000 centipoise. The gravure methodcan deposit features having an average thickness of from about 1 μm toabout 25 μm and can deposit such features at a high rate of speed, suchas up to about 700 meters per minute. The gravure process also enablesthe direct formation of patterns onto the surface.

Lithographic printing methods can also be utilized. In the lithographicprocess, the inked printing plate contacts and transfers a pattern to arubber blanket and the rubber blanket contacts and transfers the patternto the surface being printed. A plate cylinder first comes into contactwith dampening rollers that transfer an aqueous solution to thehydrophilic non-image areas of the plate. A dampened plate then contactsan inking roller and accepts the ink only in the oleophillic imageareas.

The aforementioned deposition/printing techniques may require one ormore subsequent drying and/or curing (e.g., heating) steps, such as bythermal, ultraviolet and/or infrared radiation, to induce a chemical orphysical bond formation. For example, if a long chain fluoricsubstituted aryl is used, the resulting deposited layer can be dried(e.g., at 100° C.) and heated (e.g., 350° C.) to induce mobility andphysical bond formation between adjacent modified carbon productsthrough a surface substituted aryl group.

By way of illustration, a low viscosity modified carbon ink including amodified carbon product having a hydrophobic surface group can bedeposited using a direct-write deposition tool (e.g., an ink jetprinter) to form a hydrophobic layer. After the deposited layer is dried(e.g., at about 100° C.), it can be heated (e.g., at about 350° C.) fora certain period of time (e.g., 30 minutes) to enable the hydrophobicgroups to become mobile and intertwine with adjacent surface groups onthe same and different carbon particles, thereby resulting in ahydrophobic layer with a greater level of structural integrity.

Using one or more of the foregoing deposition techniques, it is possibleto deposit modified carbon inks on one side or both sides of a surfaceto form and/or modify a proton exchange membrane. According to oneembodiment, such deposition techniques are utilized to modify anexisting proton exchange membrane. In another embodiment, suchdeposition techniques are used to directly create the proton exchangemembrane, such as by depositing a modified carbon ink onto anothercomponent of the MEA.

It will be appreciated that any of the above-noted processes can beutilized in parallel or serial to deposit multiple layers of the same ordifferent modified carbon inks onto a surface, and can be printed in oneor more dimensions and in single or multiple deposition steps. In thisregard, one embodiment of the present invention is directed to printingmultiple layers of modified carbon inks to generate gradients in theproton exchange membrane.

In one particular embodiment, gradient structures can be prepared thathave material properties that transition from very hydrophilic to veryhydrophobic, such as by utilizing a plurality of layers includingmodified carbon products or modified carbon products having varyingconcentrations of surface groups. In this regard, a first layer mayinclude a modified carbon product that is very hydrophilic, such as amodified carbon product having a hydrophilic terminated surface groupattached to the surface (e.g., a sulfuric group). On this first layer, asecond, slightly less hydrophilic layer can be formed, such as by usinga modified carbon product that has slightly hydrophilic surface groups(e.g., a carboxylic group). A third, hydrophobic layer can be formed onthe second layer utilizing a hydrophobic modified carbon product havinga hydrophobic surface group. It will be appreciated that in any of theselayers, more than one type of surface group can be utilized with thevarious modified carbon products.

The incorporation of modified carbon products in a modified carbon inkcan also affect the drying characteristics of ink. For example, rapiddrying can result in crack formation after deposition. Drying can beslowed by utilizing modified carbon products miscible with the solventto reduce the vapor pressure of the solvent after deposition. This canbe achieved by increased the solids loading of the modified carbonproducts in the modified carbon ink. In one preferred embodiment, themodified carbon ink has a solids loading of up to about 70 wt. %.Increased solids loading of modified carbon produced results in moreuniform drying and less volume fraction of solvents being removed duringdrying process. In addition, modified carbon products can include a longchain surface group (e.g., polymeric) that can form physical and/orchemical bonds to the solvent species (e.g., water, isopropanol orTEFLON) or adjacent surface groups, resulting in more uniform drying asdepicted in FIG. 12, where 1271 is a deposition process using aconventional ink and 1272 is a deposition processing using a modifiedcarbon ink.

EXAMPLES Proton Exchange Membrane Examples

A. Production of PEG-VULCAN XC-72-Reinforced PFSA PEM

90 mL of deionized water, 26.5 g of a treating agent (aminophenylatedpolyethylene glycol ether, MW˜2119 and having the formula(H₂N—C₆H₄—CO—[O—(C₂H₄O)_(n)—CH₃])) and 2.25 g of a 70% aqueous solutionof nitric acid are added to a beaker and slowly mixed. The temperatureis slowly raised to 40° C. using a hot plate. When the temperaturereaches 40° C., 10 g of VULCAN XC-72 carbon black is added and themixture is stirred and heated to 50° C. When the temperature reaches 50°C., 4.3 g of a 20 wt. % aqueous sodium nitrite solution is added slowlydrop wise. The mixture is then allowed to react at 50° C. for 2 hours.When the reaction is substantially complete, the sample is diafilteredusing 10 volumes of fresh deionized water to remove any reactionby-products. The resulting PEG-modified VULCAN XC-72 carbon is added toa PFSA water/isopropanol solution to give a modified carbon solidsloading of 55 wt. %. The slurry is cast and dried at 80° C. overnight toform a membrane.

B. Increased Proton Conductivity Composite Modified Membrane

Example B-1 To Cast a 70 wt. % SO₃H Modified Carbon Black/PFSA Membrane

To 70 g of (C₆H₄)SO₃H modified VULCAN XC-72 (3 μmols/m²) is added 2000 gof a 5 wt. % PFSA isopropanol/water solution to give a 70 wt % modifiedcarbon black/PFSA suspension. The resulting viscous suspension is mixedfor several minutes and poured onto a Teflonized glass plate. Theresulting film is doctor bladed to a thickness (wet) of 5 mils. Theresulting film is dried in air at room temperature for 24 hours.

Example B-2 To Cast a 15 wt. % SO₃H Modified Carbon Black/PFSA Membrane

To 15 g of (C₆H₄)SO₃H modified VULCAN XC-72 (3 μmols/m²) is added 2000 gof a 5 wt. % PFSA isopropanol/water solution to give a 15 wt % modifiedcarbon black/PFSA suspension. The resulting suspension is mixed forseveral minutes and poured onto a Teflonized glass plate. The resultingfilm is doctor bladed to a thickness (wet) of 20 mils. The resultingfilm is dried in air at room temperature for 24 hours. The resultingmembrane has increased mechanical stability and higher protonconductivity at lower humidification levels.

C. Direct Printed Modified Carbon Product/PFSA PEM

A modified carbon suspension containing modified carbon black includesfrom about 2 to 10 wt. % solids loading of the carbon black, ahumectant, viscosity, surface tension modifier and/or a biocide.CAB-O-JET 200 (Cabot Corporation, Boston, Mass.) is a dispersion of 20wt. % BLACKPEARLS 700, modified with (C₆H₄)SO₃H, in water. Thedispersion has a viscosity of 3.8 cP with a surface tension of 75.5dynes/cm. The acceptable viscosity for ink-jetting with the Spectraink-jet heads is about 12 cP with a surface tension of about 30dynes/cm.

Table VII illustrates an example of a low viscosity, ink-jettablemodified carbon black/PFSA ink composition. TABLE VII Component Wt. %Modified carbon 5.9 black PFSA 52.4 Isopropyl 12.6 alcohol Water 29.1Total 100.0

The formulation is ink jet printed on the electrode side of a fluiddiffusion electrode with a Spectra ink-jet head. The ink-jet headtemperature was set at 30° C., fire pulse width of 8 μs, pulse rise andfall time of 3 μs, and firing voltage of 120 V. An ink-jet print isobtained at a speed of 10 inches/sec. The printed film is dried at 80°C. for 3 hours and the printing and drying process is repeated twoadditional times.

D. Modified Polyimide-Based PEM

A mixture of C₆H₄SO₃H surface functionalized VULCAN XC-72 is mixed withfinely divided polyimide to give a final composition 70 wt. % modifiedcarbon black. The mixture is blended in a ball mill for 2 hours and isthen suspended in a water/ethanol solvent to form a slurry. The slurryis cast and dried overnight at 80° C. The resulting film is pressed to athickness of 0.12 mm and heated to a temperature of 250° C. for 2 hours.The resulting modified carbon black polyimide composite membrane hasstructural integrity and strength and is proton conducting with andwithout humidification.

E. Printed Modified Carbon Product-Based PEM

A powder batch of VULCAN XC-72 modified with (C₆H₄)SO₃H is suspended inwater at a concentration of 5 wt. %. To this suspension is added VULCANXC-72 that has been modified with polyethylene glycol (PEG), where theratio of PEG modified carbon to (C₆H₄)SO₃H modified carbon is 5:1 andthe total ink solids loading is 5 wt. %. The resulting ink is ink jetprinted directly onto an electrode layer, which is supported by a fluiddiffusion layer (e.g., a gas diffusion electrode). The printed layer isthen dried and heated at 130° C. for 2 hours to remove the solvent andbind the carbon black particles together via the PEG surface molecules.The printing, drying and heating process is repeated two more times.

F. Ink Jet Printed Modified Carbon Product-Based PEM

VULCAN XC-72 that has been co-modified with PEG and (C₆H₄)SO₃H in a 5:1wt. ratio is dispersed into an isopropanol/water solvent with a totalsolids content of 5 wt. %. The resulting ink is printed by means of apiezoelectric, drop-on-demand GALAXY ink-jet head (Spectra Corporation)in three passes. The resulting layer is heated at 120° C. for 30 minutesto allow the PEG surface groups to intertwine and to remove the solvent.

G. Ink Jet Printed Modified Carbon Product/TEFLON PEM

To VULCAN XC-72 modified with (C₆H₄)SO₃H groups (3 μmol/m²) is added aNAFION/isopropanol-water solution to give an ink that consists of 5 wt.% carbon where the carbon:NAFION weight ratio is 1:5. The resultingcarbon black/NAFION ink is ink jet printed utilizing a piezoelectricdrop on demand Galaxy ink jet head (Spectra Corporation) onto theelectrode side of a catalyst coated fluid diffusion layer and issubsequently dried at 25° C. for 12 hours. The printing and dryingprocess is repeated two more times.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations to thoseembodiments will occur to those skilled in the art. It is to beexpressly understood, however, that such modifications and adaptationsare within the scope and spirit of the present invention, as set forthin the claims below. Further, it should be recognized that any featureof any embodiment disclosed herein can be combined with any otherfeature of any other embodiment in any combination.

1. A proton conducting membrane, said membrane comprising a modifiedcarbon product and a polymer.
 2. A proton conducting membrane as recitedin claim 1, wherein said modified carbon product comprises modifiedcarbon black.
 3. A proton conducting membrane as recited in claim 1,wherein said modified carbon product comprises a proton conductingfunctional group.
 4. A proton conducting membrane as recited in any ofclaim 1, wherein said polymer is selected from the group consisting ofsulfonated PTFE and perfluorosulfonated PTFE.
 5. A proton conductingmembrane as recited in claim 1, wherein said polymer is selected fromthe group consisting of polyvinylidene fluoride (PVDF), acid-doped orderivatized hydrocarbon polymers, such as polybenzimidizole (PBI),polyarylenes, polyetherketones, polysulfones, phosphazenes andpolyimides.
 6. A proton conducting membrane as recited in claim 1,wherein said modified carbon product is coated on a surface of saidpolymer.
 7. A proton conducting membrane as recited in claim 1, whereinsaid modified carbon product is dispersed within said polymer.
 8. Aproton conducting membrane as recited in claim 1, wherein saidproton-conducting membrane has a proton conducting group concentrationof at least about 5.0 mmol/mL.
 9. A proton conducting membrane asrecited in claim 1, wherein said proton-conducting membrane has a protonconducting group concentration of at least about 5.4 mmol/mL.
 10. Aproton conducting membrane as recited in claim 1, wherein said modifiedcarbon product is adapted to conduct protons in the absence of water.11. A proton conducting membrane as recited in claim 1, wherein saidmodified carbon product is adapted to selectively conduct protons in thepresence of other hydrogen-comprising liquid fuels.
 12. A protonconducting membrane as recited in claim 1, wherein said modified carbonproduct is adapted to selectively conduct protons in the presence ofmethanol or ethanol.
 13. A proton conducting membrane as recited inclaim 1, wherein said modified carbon product is adapted to yield anincreased mechanical strength without a substantial decrease in protonconductivity.
 14. A proton conducting membrane as recited in claim 1,wherein said modified carbon product comprises at least one protonconducting functional group selected from the group consisting of SO₃H,CO₂H, PO₃H₂ and PO₃ MH, where M is a monovalent cation.
 15. A protonconducting membrane, wherein said proton conducting membrane consistsessentially of a modified carbon product.
 16. A proton conductingmembrane as recited in claim 15, wherein said modified carbon productcomprises proton-conducting functional groups.
 17. A proton conductingmembrane as recited in claim 15, wherein said modified carbon productcomprises proton-conducting functional groups selected from the groupconsisting of carboxylic acids, sulfonic acids, phosphonic acids andphosphonic acid salts.
 18. A proton conducting membrane as recited inclaim 15, wherein said modified carbon product comprises modified carbonblack.
 19. A proton conducting membrane as recited in claim 15, whereinsaid modified carbon product comprises modified carbon fibers.
 20. Amethod for the fabrication of a proton conducting membrane, comprisingthe steps of: a) mixing a polymer with a modified carbon product to forma composite mixture; and b) forming said composite mixture into a protonconducting membrane.
 21. A method as recited in claim 20, wherein saidforming step comprises extruding said composite mixture.
 22. A method asrecited in claim 20, wherein said forming step comprises casting saidcomposite mixture.
 23. A method as recited in claim 20, wherein saidproton conducting membrane has a volume density of proton conductinggroups of at least about 4.8 mmol/mL.
 24. A method as recited in claim20, wherein said composite mixture comprises at least about 20 wt. %modified carbon product.
 25. A method as recited in claim 20, whereinsaid proton conducting membrane comprises at least about 40 wt. %modified carbon product.
 26. A method as recited in claim 20, whereinsaid carbon product comprises carbon black.
 27. A method as recited inclaim 20, wherein said carbon product comprises carbon fibers.
 28. Amethod for the fabrication of a proton conducting membrane, comprisingthe steps of: a) providing a modified carbon black product; and b)forming said modified carbon black product into a thin membrane.
 29. Amethod as recited in claim 28, wherein said forming step comprisesanalog deposition.
 30. A method as recited in claim 28, wherein saidforming step comprises digital deposition.
 31. A method as recited inclaim 28, wherein said forming step comprises dispersing said modifiedcarbon product in a liquid vehicle and ink-jet printing said modifiedcarbon product.
 32. A method as recited in claim 31, wherein saidmodified carbon product comprises hydrophilic functional groups.